coding_ddpg.py

# -*- coding: utf-8 -*-
"""
TorchRL objectives: Coding a DDPG loss
======================================
**Author**: `Vincent Moens <https://github.com/vmoens>`_

"""

##############################################################################
# Overview
# --------
#
# TorchRL separates the training of RL algorithms in various pieces that will be
# assembled in your training script: the environment, the data collection and
# storage, the model and finally the loss function.
#
# TorchRL losses (or "objectives") are stateful objects that contain the
# trainable parameters (policy and value models).
# This tutorial will guide you through the steps to code a loss from the ground up
# using TorchRL.
#
# To this aim, we will be focusing on DDPG, which is a relatively straightforward
# algorithm to code.
# `Deep Deterministic Policy Gradient <https://arxiv.org/abs/1509.02971>`_ (DDPG)
# is a simple continuous control algorithm. It consists in learning a
# parametric value function for an action-observation pair, and
# then learning a policy that outputs actions that maximize this value
# function given a certain observation.
#
# What you will learn:
#
# - how to write a loss module and customize its value estimator;
# - how to build an environment in TorchRL, including transforms
#   (for example, data normalization) and parallel execution;
# - how to design a policy and value network;
# - how to collect data from your environment efficiently and store them
#   in a replay buffer;
# - how to store trajectories (and not transitions) in your replay buffer);
# - how to evaluate your model.
#
# Prerequisites
# ~~~~~~~~~~~~~
#
# This tutorial assumes that you have completed the
# `PPO tutorial <reinforcement_ppo.html>`_ which gives
# an overview of the TorchRL components and dependencies, such as
# :class:`tensordict.TensorDict` and :class:`tensordict.nn.TensorDictModules`,
# although it should be
# sufficiently transparent to be understood without a deep understanding of
# these classes.
#
# .. note::
#   We do not aim at giving a SOTA implementation of the algorithm, but rather
#   to provide a high-level illustration of TorchRL's loss implementations
#   and the library features that are to be used in the context of
#   this algorithm.
#
# Imports and setup
# -----------------
#
#  .. code-block:: bash
#
#      %%bash
#      pip3 install torchrl mujoco glfw

# sphinx_gallery_start_ignore
import warnings

warnings.filterwarnings("ignore")
from torch import multiprocessing

# TorchRL prefers spawn method, that restricts creation of  ``~torchrl.envs.ParallelEnv`` inside
# `__main__` method call, but for the easy of reading the code switch to fork
# which is also a default spawn method in Google's Colaboratory
try:
    multiprocessing.set_start_method("fork")
except RuntimeError:
    pass

# sphinx_gallery_end_ignore


import torch
import tqdm


###############################################################################
# We will execute the policy on CUDA if available
is_fork = multiprocessing.get_start_method() == "fork"
device = (
    torch.device(0)
    if torch.cuda.is_available() and not is_fork
    else torch.device("cpu")
)
collector_device = torch.device("cpu")  # Change the device to ``cuda`` to use CUDA

###############################################################################
# TorchRL :class:`~torchrl.objectives.LossModule`
# -----------------------------------------------
#
# TorchRL provides a series of losses to use in your training scripts.
# The aim is to have losses that are easily reusable/swappable and that have
# a simple signature.
#
# The main characteristics of TorchRL losses are:
#
# - They are stateful objects: they contain a copy of the trainable parameters
#   such that ``loss_module.parameters()`` gives whatever is needed to train the
#   algorithm.
# - They follow the ``TensorDict`` convention: the :meth:`torch.nn.Module.forward`
#   method will receive a TensorDict as input that contains all the necessary
#   information to return a loss value.
#
#       >>> data = replay_buffer.sample()
#       >>> loss_dict = loss_module(data)
#
# - They output a :class:`tensordict.TensorDict` instance with the loss values
#   written under a ``"loss_<smth>"`` where ``smth`` is a string describing the
#   loss. Additional keys in the ``TensorDict`` may be useful metrics to log during
#   training time.
#
#   .. note::
#     The reason we return independent losses is to let the user use a different
#     optimizer for different sets of parameters for instance. Summing the losses
#     can be simply done via
#
#       >>> loss_val = sum(loss for key, loss in loss_dict.items() if key.startswith("loss_"))
#
# The ``__init__`` method
# ~~~~~~~~~~~~~~~~~~~~~~~
#
# The parent class of all losses is :class:`~torchrl.objectives.LossModule`.
# As many other components of the library, its :meth:`~torchrl.objectives.LossModule.forward` method expects
# as input a :class:`tensordict.TensorDict` instance sampled from an experience
# replay buffer, or any similar data structure. Using this format makes it
# possible to re-use the module across
# modalities, or in complex settings where the model needs to read multiple
# entries for instance. In other words, it allows us to code a loss module that
# is oblivious to the data type that is being given to is and that focuses on
# running the elementary steps of the loss function and only those.
#
# To keep the tutorial as didactic as we can, we'll be displaying each method
# of the class independently and we'll be populating the class at a later
# stage.
#
# Let us start with the :meth:`~torchrl.objectives.LossModule.__init__`
# method. DDPG aims at solving a control task with a simple strategy:
# training a policy to output actions that maximize the value predicted by
# a value network. Hence, our loss module needs to receive two networks in its
# constructor: an actor and a value networks. We expect both of these to be
# TensorDict-compatible objects, such as
# :class:`tensordict.nn.TensorDictModule`.
# Our loss function will need to compute a target value and fit the value
# network to this, and generate an action and fit the policy such that its
# value estimate is maximized.
#
# The crucial step of the :meth:`LossModule.__init__` method is the call to
# :meth:`~torchrl.LossModule.convert_to_functional`. This method will extract
# the parameters from the module and convert it to a functional module.
# Strictly speaking, this is not necessary and one may perfectly code all
# the losses without it. However, we encourage its usage for the following
# reason.
#
# The reason TorchRL does this is that RL algorithms often execute the same
# model with different sets of parameters, called "trainable" and "target"
# parameters.
# The "trainable" parameters are those that the optimizer needs to fit. The
# "target" parameters are usually a copy of the former's with some time lag
# (absolute or diluted through a moving average).
# These target parameters are used to compute the value associated with the
# next observation. One the advantages of using a set of target parameters
# for the value model that do not match exactly the current configuration is
# that they provide a pessimistic bound on the value function being computed.
# Pay attention to the ``create_target_params`` keyword argument below: this
# argument tells the :meth:`~torchrl.objectives.LossModule.convert_to_functional`
# method to create a set of target parameters in the loss module to be used
# for target value computation. If this is set to ``False`` (see the actor network
# for instance) the ``target_actor_network_params`` attribute will still be
# accessible but this will just return a **detached** version of the
# actor parameters.
#
# Later, we will see how the target parameters should be updated in TorchRL.
#

from tensordict.nn import TensorDictModule, TensorDictSequential


def _init(
    self,
    actor_network: TensorDictModule,
    value_network: TensorDictModule,
) -> None:
    super(type(self), self).__init__()

    self.convert_to_functional(
        actor_network,
        "actor_network",
        create_target_params=True,
    )
    self.convert_to_functional(
        value_network,
        "value_network",
        create_target_params=True,
        compare_against=list(actor_network.parameters()),
    )

    self.actor_in_keys = actor_network.in_keys

    # Since the value we'll be using is based on the actor and value network,
    # we put them together in a single actor-critic container.
    actor_critic = ActorCriticWrapper(actor_network, value_network)
    self.actor_critic = actor_critic
    self.loss_function = "l2"


###############################################################################
# The value estimator loss method
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# In many RL algorithm, the value network (or Q-value network) is trained based
# on an empirical value estimate. This can be bootstrapped (TD(0), low
# variance, high bias), meaning
# that the target value is obtained using the next reward and nothing else, or
# a Monte-Carlo estimate can be obtained (TD(1)) in which case the whole
# sequence of upcoming rewards will be used (high variance, low bias). An
# intermediate estimator (TD(:math:`\lambda`)) can also be used to compromise
# bias and variance.
# TorchRL makes it easy to use one or the other estimator via the
# :class:`~torchrl.objectives.utils.ValueEstimators` Enum class, which contains
# pointers to all the value estimators implemented. Let us define the default
# value function here. We will take the simplest version (TD(0)), and show later
# on how this can be changed.

from torchrl.objectives.utils import ValueEstimators

default_value_estimator = ValueEstimators.TD0

###############################################################################
# We also need to give some instructions to DDPG on how to build the value
# estimator, depending on the user query. Depending on the estimator provided,
# we will build the corresponding module to be used at train time:

from torchrl.objectives.utils import default_value_kwargs
from torchrl.objectives.value import TD0Estimator, TD1Estimator, TDLambdaEstimator


def make_value_estimator(self, value_type: ValueEstimators, **hyperparams):
    hp = dict(default_value_kwargs(value_type))
    if hasattr(self, "gamma"):
        hp["gamma"] = self.gamma
    hp.update(hyperparams)
    value_key = "state_action_value"
    if value_type == ValueEstimators.TD1:
        self._value_estimator = TD1Estimator(value_network=self.actor_critic, **hp)
    elif value_type == ValueEstimators.TD0:
        self._value_estimator = TD0Estimator(value_network=self.actor_critic, **hp)
    elif value_type == ValueEstimators.GAE:
        raise NotImplementedError(
            f"Value type {value_type} it not implemented for loss {type(self)}."
        )
    elif value_type == ValueEstimators.TDLambda:
        self._value_estimator = TDLambdaEstimator(value_network=self.actor_critic, **hp)
    else:
        raise NotImplementedError(f"Unknown value type {value_type}")
    self._value_estimator.set_keys(value=value_key)


###############################################################################
# The ``make_value_estimator`` method can but does not need to be called: if
# not, the :class:`~torchrl.objectives.LossModule` will query this method with
# its default estimator.
#
# The actor loss method
# ~~~~~~~~~~~~~~~~~~~~~
#
# The central piece of an RL algorithm is the training loss for the actor.
# In the case of DDPG, this function is quite simple: we just need to compute
# the value associated with an action computed using the policy and optimize
# the actor weights to maximize this value.
#
# When computing this value, we must make sure to take the value parameters out
# of the graph, otherwise the actor and value loss will be mixed up.
# For this, the :func:`~torchrl.objectives.utils.hold_out_params` function
# can be used.


def _loss_actor(
    self,
    tensordict,
) -> torch.Tensor:
    td_copy = tensordict.select(*self.actor_in_keys)
    # Get an action from the actor network: since we made it functional, we need to pass the params
    with self.actor_network_params.to_module(self.actor_network):
        td_copy = self.actor_network(td_copy)
    # get the value associated with that action
    with self.value_network_params.detach().to_module(self.value_network):
        td_copy = self.value_network(td_copy)
    return -td_copy.get("state_action_value")


###############################################################################
# The value loss method
# ~~~~~~~~~~~~~~~~~~~~~
#
# We now need to optimize our value network parameters.
# To do this, we will rely on the value estimator of our class:
#

from torchrl.objectives.utils import distance_loss


def _loss_value(
    self,
    tensordict,
):
    td_copy = tensordict.clone()

    # V(s, a)
    with self.value_network_params.to_module(self.value_network):
        self.value_network(td_copy)
    pred_val = td_copy.get("state_action_value").squeeze(-1)

    # we manually reconstruct the parameters of the actor-critic, where the first
    # set of parameters belongs to the actor and the second to the value function.
    target_params = TensorDict(
        {
            "module": {
                "0": self.target_actor_network_params,
                "1": self.target_value_network_params,
            }
        },
        batch_size=self.target_actor_network_params.batch_size,
        device=self.target_actor_network_params.device,
    )
    with target_params.to_module(self.actor_critic):
        target_value = self.value_estimator.value_estimate(tensordict).squeeze(-1)

    # Computes the value loss: L2, L1 or smooth L1 depending on `self.loss_function`
    loss_value = distance_loss(pred_val, target_value, loss_function=self.loss_function)
    td_error = (pred_val - target_value).pow(2)

    return loss_value, td_error, pred_val, target_value


###############################################################################
# Putting things together in a forward call
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# The only missing piece is the forward method, which will glue together the
# value and actor loss, collect the cost values and write them in a ``TensorDict``
# delivered to the user.

from tensordict import TensorDict, TensorDictBase


def _forward(self, input_tensordict: TensorDictBase) -> TensorDict:
    loss_value, td_error, pred_val, target_value = self.loss_value(
        input_tensordict,
    )
    td_error = td_error.detach()
    td_error = td_error.unsqueeze(input_tensordict.ndimension())
    if input_tensordict.device is not None:
        td_error = td_error.to(input_tensordict.device)
    input_tensordict.set(
        "td_error",
        td_error,
        inplace=True,
    )
    loss_actor = self.loss_actor(input_tensordict)
    return TensorDict(
        source={
            "loss_actor": loss_actor.mean(),
            "loss_value": loss_value.mean(),
            "pred_value": pred_val.mean().detach(),
            "target_value": target_value.mean().detach(),
            "pred_value_max": pred_val.max().detach(),
            "target_value_max": target_value.max().detach(),
        },
        batch_size=[],
    )


from torchrl.objectives import LossModule


class DDPGLoss(LossModule):
    default_value_estimator = default_value_estimator
    make_value_estimator = make_value_estimator

    __init__ = _init
    forward = _forward
    loss_value = _loss_value
    loss_actor = _loss_actor


###############################################################################
# Now that we have our loss, we can use it to train a policy to solve a
# control task.
#
# Environment
# -----------
#
# In most algorithms, the first thing that needs to be taken care of is the
# construction of the environment as it conditions the remainder of the
# training script.
#
# For this example, we will be using the ``"cheetah"`` task. The goal is to make
# a half-cheetah run as fast as possible.
#
# In TorchRL, one can create such a task by relying on ``dm_control`` or ``gym``:
#
# .. code-block:: python
#
#    env = GymEnv("HalfCheetah-v4")
#
# or
#
# .. code-block:: python
#
#    env = DMControlEnv("cheetah", "run")
#
# By default, these environment disable rendering. Training from states is
# usually easier than training from images. To keep things simple, we focus
# on learning from states only. To pass the pixels to the ``tensordicts`` that
# are collected by :func:`env.step()`, simply pass the ``from_pixels=True``
# argument to the constructor:
#
# .. code-block:: python
#
#    env = GymEnv("HalfCheetah-v4", from_pixels=True, pixels_only=True)
#
# We write a :func:`make_env` helper function that will create an environment
# with either one of the two backends considered above (``dm-control`` or ``gym``).
#

from torchrl.envs.libs.dm_control import DMControlEnv
from torchrl.envs.libs.gym import GymEnv

env_library = None
env_name = None


def make_env(from_pixels=False):
    """Create a base ``env``."""
    global env_library
    global env_name

    if backend == "dm_control":
        env_name = "cheetah"
        env_task = "run"
        env_args = (env_name, env_task)
        env_library = DMControlEnv
    elif backend == "gym":
        env_name = "HalfCheetah-v4"
        env_args = (env_name,)
        env_library = GymEnv
    else:
        raise NotImplementedError

    env_kwargs = {
        "device": device,
        "from_pixels": from_pixels,
        "pixels_only": from_pixels,
        "frame_skip": 2,
    }
    env = env_library(*env_args, **env_kwargs)
    return env


###############################################################################
# Transforms
# ~~~~~~~~~~
#
# Now that we have a base environment, we may want to modify its representation
# to make it more policy-friendly. In TorchRL, transforms are appended to the
# base environment in a specialized :class:`torchr.envs.TransformedEnv` class.
#
# - It is common in DDPG to rescale the reward using some heuristic value. We
#   will multiply the reward by 5 in this example.
#
# - If we are using :mod:`dm_control`, it is also important to build an interface
#   between the simulator which works with double precision numbers, and our
#   script which presumably uses single precision ones. This transformation goes
#   both ways: when calling :func:`env.step`, our actions will need to be
#   represented in double precision, and the output will need to be transformed
#   to single precision.
#   The :class:`~torchrl.envs.DoubleToFloat` transform does exactly this: the
#   ``in_keys`` list refers to the keys that will need to be transformed from
#   double to float, while the ``in_keys_inv`` refers to those that need to
#   be transformed to double before being passed to the environment.
#
# - We concatenate the state keys together using the :class:`~torchrl.envs.CatTensors`
#   transform.
#
# - Finally, we also leave the possibility of normalizing the states: we will
#   take care of computing the normalizing constants later on.
#

from torchrl.envs import (
    CatTensors,
    DoubleToFloat,
    EnvCreator,
    InitTracker,
    ObservationNorm,
    ParallelEnv,
    RewardScaling,
    StepCounter,
    TransformedEnv,
)


def make_transformed_env(
    env,
):
    """Apply transforms to the ``env`` (such as reward scaling and state normalization)."""

    env = TransformedEnv(env)

    # we append transforms one by one, although we might as well create the
    # transformed environment using the `env = TransformedEnv(base_env, transforms)`
    # syntax.
    env.append_transform(RewardScaling(loc=0.0, scale=reward_scaling))

    # We concatenate all states into a single "observation_vector"
    # even if there is a single tensor, it'll be renamed in "observation_vector".
    # This facilitates the downstream operations as we know the name of the
    # output tensor.
    # In some environments (not half-cheetah), there may be more than one
    # observation vector: in this case this code snippet will concatenate them
    # all.
    selected_keys = list(env.observation_spec.keys())
    out_key = "observation_vector"
    env.append_transform(CatTensors(in_keys=selected_keys, out_key=out_key))

    # we normalize the states, but for now let's just instantiate a stateless
    # version of the transform
    env.append_transform(ObservationNorm(in_keys=[out_key], standard_normal=True))

    env.append_transform(DoubleToFloat())

    env.append_transform(StepCounter(max_frames_per_traj))

    # We need a marker for the start of trajectories for our Ornstein-Uhlenbeck (OU)
    # exploration:
    env.append_transform(InitTracker())

    return env


###############################################################################
# Parallel execution
# ~~~~~~~~~~~~~~~~~~
#
# The following helper function allows us to run environments in parallel.
# Running environments in parallel can significantly speed up the collection
# throughput. When using transformed environment, we need to choose whether we
# want to execute the transform individually for each environment, or
# centralize the data and transform it in batch. Both approaches are easy to
# code:
#
# .. code-block:: python
#
#    env = ParallelEnv(
#        lambda: TransformedEnv(GymEnv("HalfCheetah-v4"), transforms),
#        num_workers=4
#    )
#    env = TransformedEnv(
#        ParallelEnv(lambda: GymEnv("HalfCheetah-v4"), num_workers=4),
#        transforms
#    )
#
# To leverage the vectorization capabilities of PyTorch, we adopt
# the first method:
#


def parallel_env_constructor(
    env_per_collector,
    transform_state_dict,
):
    if env_per_collector == 1:

        def make_t_env():
            env = make_transformed_env(make_env())
            env.transform[2].init_stats(3)
            env.transform[2].loc.copy_(transform_state_dict["loc"])
            env.transform[2].scale.copy_(transform_state_dict["scale"])
            return env

        env_creator = EnvCreator(make_t_env)
        return env_creator

    parallel_env = ParallelEnv(
        num_workers=env_per_collector,
        create_env_fn=EnvCreator(lambda: make_env()),
        create_env_kwargs=None,
        pin_memory=False,
    )
    env = make_transformed_env(parallel_env)
    # we call `init_stats` for a limited number of steps, just to instantiate
    # the lazy buffers.
    env.transform[2].init_stats(3, cat_dim=1, reduce_dim=[0, 1])
    env.transform[2].load_state_dict(transform_state_dict)
    return env


# The backend can be ``gym`` or ``dm_control``
backend = "gym"

###############################################################################
# .. note::
#
#   ``frame_skip`` batches multiple step together with a single action
#   If > 1, the other frame counts (for example, frames_per_batch, total_frames)
#   need to be adjusted to have a consistent total number of frames collected
#   across experiments. This is important as raising the frame-skip but keeping the
#   total number of frames unchanged may seem like cheating: all things compared,
#   a dataset of 10M elements collected with a frame-skip of 2 and another with
#   a frame-skip of 1 actually have a ratio of interactions with the environment
#   of 2:1! In a nutshell, one should be cautious about the frame-count of a
#   training script when dealing with frame skipping as this may lead to
#   biased comparisons between training strategies.
#
# Scaling the reward helps us control the signal magnitude for a more
# efficient learning.
reward_scaling = 5.0

###############################################################################
# We also define when a trajectory will be truncated. A thousand steps (500 if
# frame-skip = 2) is a good number to use for the cheetah task:

max_frames_per_traj = 500

###############################################################################
# Normalization of the observations
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# To compute the normalizing statistics, we run an arbitrary number of random
# steps in the environment and compute the mean and standard deviation of the
# collected observations. The :func:`ObservationNorm.init_stats()` method can
# be used for this purpose. To get the summary statistics, we create a dummy
# environment and run it for a given number of steps, collect data over a given
# number of steps and compute its summary statistics.
#


def get_env_stats():
    """Gets the stats of an environment."""
    proof_env = make_transformed_env(make_env())
    t = proof_env.transform[2]
    t.init_stats(init_env_steps)
    transform_state_dict = t.state_dict()
    proof_env.close()
    return transform_state_dict


###############################################################################
# Normalization stats
# ~~~~~~~~~~~~~~~~~~~
# Number of random steps used as for stats computation using ``ObservationNorm``

init_env_steps = 5000

transform_state_dict = get_env_stats()

###############################################################################
# Number of environments in each data collector
env_per_collector = 4

###############################################################################
# We pass the stats computed earlier to normalize the output of our
# environment:

parallel_env = parallel_env_constructor(
    env_per_collector=env_per_collector,
    transform_state_dict=transform_state_dict,
)


from torchrl.data import CompositeSpec

###############################################################################
# Building the model
# ------------------
#
# We now turn to the setup of the model. As we have seen, DDPG requires a
# value network, trained to estimate the value of a state-action pair, and a
# parametric actor that learns how to select actions that maximize this value.
#
# Recall that building a TorchRL module requires two steps:
#
# - writing the :class:`torch.nn.Module` that will be used as network,
# - wrapping the network in a :class:`tensordict.nn.TensorDictModule` where the
#   data flow is handled by specifying the input and output keys.
#
# In more complex scenarios, :class:`tensordict.nn.TensorDictSequential` can
# also be used.
#
#
# The Q-Value network is wrapped in a :class:`~torchrl.modules.ValueOperator`
# that automatically sets the ``out_keys`` to ``"state_action_value`` for q-value
# networks and ``state_value`` for other value networks.
#
# TorchRL provides a built-in version of the DDPG networks as presented in the
# original paper. These can be found under :class:`~torchrl.modules.DdpgMlpActor`
# and :class:`~torchrl.modules.DdpgMlpQNet`.
#
# Since we use lazy modules, it is necessary to materialize the lazy modules
# before being able to move the policy from device to device and achieve other
# operations. Hence, it is good practice to run the modules with a small
# sample of data. For this purpose, we generate fake data from the
# environment specs.
#

from torchrl.modules import (
    ActorCriticWrapper,
    DdpgMlpActor,
    DdpgMlpQNet,
    OrnsteinUhlenbeckProcessModule,
    ProbabilisticActor,
    TanhDelta,
    ValueOperator,
)


def make_ddpg_actor(
    transform_state_dict,
    device="cpu",
):
    proof_environment = make_transformed_env(make_env())
    proof_environment.transform[2].init_stats(3)
    proof_environment.transform[2].load_state_dict(transform_state_dict)

    out_features = proof_environment.action_spec.shape[-1]

    actor_net = DdpgMlpActor(
        action_dim=out_features,
    )

    in_keys = ["observation_vector"]
    out_keys = ["param"]

    actor = TensorDictModule(
        actor_net,
        in_keys=in_keys,
        out_keys=out_keys,
    )

    actor = ProbabilisticActor(
        actor,
        distribution_class=TanhDelta,
        in_keys=["param"],
        spec=CompositeSpec(action=proof_environment.action_spec),
    ).to(device)

    q_net = DdpgMlpQNet()

    in_keys = in_keys + ["action"]
    qnet = ValueOperator(
        in_keys=in_keys,
        module=q_net,
    ).to(device)

    # initialize lazy modules
    qnet(actor(proof_environment.reset().to(device)))
    return actor, qnet


actor, qnet = make_ddpg_actor(
    transform_state_dict=transform_state_dict,
    device=device,
)

###############################################################################
# Exploration
# ~~~~~~~~~~~
#
# The policy is passed into a :class:`~torchrl.modules.OrnsteinUhlenbeckProcessModule`
# exploration module, as suggested in the original paper.
# Let's define the number of frames before OU noise reaches its minimum value
annealing_frames = 1_000_000

actor_model_explore = TensorDictSequential(
    actor,
    OrnsteinUhlenbeckProcessModule(
        spec=actor.spec.clone(),
        annealing_num_steps=annealing_frames,
    ).to(device),
)
if device == torch.device("cpu"):
    actor_model_explore.share_memory()


###############################################################################
# Data collector
# --------------
#
# TorchRL provides specialized classes to help you collect data by executing
# the policy in the environment. These "data collectors" iteratively compute
# the action to be executed at a given time, then execute a step in the
# environment and reset it when required.
# Data collectors are designed to help developers have a tight control
# on the number of frames per batch of data, on the (a)sync nature of this
# collection and on the resources allocated to the data collection (for example
# GPU, number of workers, and so on).
#
# Here we will use
# :class:`~torchrl.collectors.SyncDataCollector`, a simple, single-process
# data collector. TorchRL offers other collectors, such as
# :class:`~torchrl.collectors.MultiaSyncDataCollector`, which executed the
# rollouts in an asynchronous manner (for example, data will be collected while
# the policy is being optimized, thereby decoupling the training and
# data collection).
#
# The parameters to specify are:
#
# - an environment factory or an environment,
# - the policy,
# - the total number of frames before the collector is considered empty,
# - the maximum number of frames per trajectory (useful for non-terminating
#   environments, like ``dm_control`` ones).
#
#   .. note::
#
#     The ``max_frames_per_traj`` passed to the collector will have the effect
#     of registering a new :class:`~torchrl.envs.StepCounter` transform
#     with the environment used for inference. We can achieve the same result
#     manually, as we do in this script.
#
# One should also pass:
#
# - the number of frames in each batch collected,
# - the number of random steps executed independently from the policy,
# - the devices used for policy execution
# - the devices used to store data before the data is passed to the main
#   process.
#
# The total frames we will use during training should be around 1M.
total_frames = 10_000  # 1_000_000

###############################################################################
# The number of frames returned by the collector at each iteration of the outer
# loop is equal to the length of each sub-trajectories times the number of
# environments run in parallel in each collector.
#
# In other words, we expect batches from the collector to have a shape
# ``[env_per_collector, traj_len]`` where
# ``traj_len=frames_per_batch/env_per_collector``:
#
traj_len = 200
frames_per_batch = env_per_collector * traj_len
init_random_frames = 5000
num_collectors = 2

from torchrl.collectors import SyncDataCollector
from torchrl.envs import ExplorationType

collector = SyncDataCollector(
    parallel_env,
    policy=actor_model_explore,
    total_frames=total_frames,
    frames_per_batch=frames_per_batch,
    init_random_frames=init_random_frames,
    reset_at_each_iter=False,
    split_trajs=False,
    device=collector_device,
    exploration_type=ExplorationType.RANDOM,
)

###############################################################################
# Evaluator: building your recorder object
# ----------------------------------------
#
# As the training data is obtained using some exploration strategy, the true
# performance of our algorithm needs to be assessed in deterministic mode. We
# do this using a dedicated class, ``Recorder``, which executes the policy in
# the environment at a given frequency and returns some statistics obtained
# from these simulations.
#
# The following helper function builds this object:
from torchrl.trainers import Recorder


def make_recorder(actor_model_explore, transform_state_dict, record_interval):
    base_env = make_env()
    environment = make_transformed_env(base_env)
    environment.transform[2].init_stats(
        3
    )  # must be instantiated to load the state dict
    environment.transform[2].load_state_dict(transform_state_dict)

    recorder_obj = Recorder(
        record_frames=1000,
        policy_exploration=actor_model_explore,
        environment=environment,
        exploration_type=ExplorationType.DETERMINISTIC,
        record_interval=record_interval,
    )
    return recorder_obj


###############################################################################
# We will be recording the performance every 10 batch collected
record_interval = 10

recorder = make_recorder(
    actor_model_explore, transform_state_dict, record_interval=record_interval
)

from torchrl.data.replay_buffers import (
    LazyMemmapStorage,
    PrioritizedSampler,
    RandomSampler,
    TensorDictReplayBuffer,
)

###############################################################################
# Replay buffer
# -------------
#
# Replay buffers come in two flavors: prioritized (where some error signal
# is used to give a higher likelihood of sampling to some items than others)
# and regular, circular experience replay.
#
# TorchRL replay buffers are composable: one can pick up the storage, sampling
# and writing strategies. It is also possible to
# store tensors on physical memory using a memory-mapped array. The following
# function takes care of creating the replay buffer with the desired
# hyperparameters:
#

from torchrl.envs import RandomCropTensorDict


def make_replay_buffer(buffer_size, batch_size, random_crop_len, prefetch=3, prb=False):
    if prb:
        sampler = PrioritizedSampler(
            max_capacity=buffer_size,
            alpha=0.7,
            beta=0.5,
        )
    else:
        sampler = RandomSampler()
    replay_buffer = TensorDictReplayBuffer(
        storage=LazyMemmapStorage(
            buffer_size,
            scratch_dir=buffer_scratch_dir,
        ),
        batch_size=batch_size,
        sampler=sampler,
        pin_memory=False,
        prefetch=prefetch,
        transform=RandomCropTensorDict(random_crop_len, sample_dim=1),
    )
    return replay_buffer


###############################################################################
# We'll store the replay buffer in a temporary directory on disk

import tempfile

tmpdir = tempfile.TemporaryDirectory()
buffer_scratch_dir = tmpdir.name

###############################################################################
# Replay buffer storage and batch size
# ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#
# TorchRL replay buffer counts the number of elements along the first dimension.
# Since we'll be feeding trajectories to our buffer, we need to adapt the buffer
# size by dividing it by the length of the sub-trajectories yielded by our
# data collector.
# Regarding the batch-size, our sampling strategy will consist in sampling
# trajectories of length ``traj_len=200`` before selecting sub-trajectories
# or length ``random_crop_len=25`` on which the loss will be computed.
# This strategy balances the choice of storing whole trajectories of a certain
# length with the need for providing samples with a sufficient heterogeneity
# to our loss. The following figure shows the dataflow from a collector
# that gets 8 frames in each batch with 2 environments run in parallel,
# feeds them to a replay buffer that contains 1000 trajectories and
# samples sub-trajectories of 2 time steps each.
#
# .. figure:: /_static/img/replaybuffer_traj.png
#    :alt: Storing trajectories in the replay buffer
#
# Let's start with the number of frames stored in the buffer


def ceil_div(x, y):
    return -x // (-y)


buffer_size = 1_000_000
buffer_size = ceil_div(buffer_size, traj_len)

###############################################################################
# Prioritized replay buffer is disabled by default
prb = False

###############################################################################
# We also need to define how many updates we'll be doing per batch of data
# collected. This is known as the update-to-data or ``UTD`` ratio:
update_to_data = 64

###############################################################################
# We'll be feeding the loss with trajectories of length 25:
random_crop_len = 25

###############################################################################
# In the original paper, the authors perform one update with a batch of 64
# elements for each frame collected. Here, we reproduce the same ratio
# but while realizing several updates at each batch collection. We
# adapt our batch-size to achieve the same number of update-per-frame ratio:

batch_size = ceil_div(64 * frames_per_batch, update_to_data * random_crop_len)

replay_buffer = make_replay_buffer(
    buffer_size=buffer_size,
    batch_size=batch_size,
    random_crop_len=random_crop_len,
    prefetch=3,
    prb=prb,
)

###############################################################################
# Loss module construction
# ------------------------
#
# We build our loss module with the actor and ``qnet`` we've just created.
# Because we have target parameters to update, we _must_ create a target network
# updater.
#

gamma = 0.99
lmbda = 0.9
tau = 0.001  # Decay factor for the target network

loss_module = DDPGLoss(actor, qnet)

###############################################################################
# let's use the TD(lambda) estimator!
loss_module.make_value_estimator(ValueEstimators.TDLambda, gamma=gamma, lmbda=lmbda, device=device)

###############################################################################
# .. note::
#   Off-policy usually dictates a TD(0) estimator. Here, we use a TD(:math:`\lambda`)
#   estimator, which will introduce some bias as the trajectory that follows
#   a certain state has been collected with an outdated policy.
#   This trick, as the multi-step trick that can be used during data collection,
#   are alternative versions of "hacks" that we usually find to work well in
#   practice despite the fact that they introduce some bias in the return
#   estimates.
#
# Target network updater
# ~~~~~~~~~~~~~~~~~~~~~~
#
# Target networks are a crucial part of off-policy RL algorithms.
# Updating the target network parameters is made easy thanks to the
# :class:`~torchrl.objectives.HardUpdate` and :class:`~torchrl.objectives.SoftUpdate`
# classes. They're built with the loss module as argument, and the update is
# achieved via a call to `updater.step()` at the appropriate location in the
# training loop.

from torchrl.objectives.utils import SoftUpdate

target_net_updater = SoftUpdate(loss_module, eps=1 - tau)

###############################################################################
# Optimizer
# ~~~~~~~~~
#
# Finally, we will use the Adam optimizer for the policy and value network:

from torch import optim

optimizer_actor = optim.Adam(
    loss_module.actor_network_params.values(True, True), lr=1e-4, weight_decay=0.0
)
optimizer_value = optim.Adam(
    loss_module.value_network_params.values(True, True), lr=1e-3, weight_decay=1e-2
)
total_collection_steps = total_frames // frames_per_batch

###############################################################################
# Time to train the policy
# ------------------------
#
# The training loop is pretty straightforward now that we have built all the
# modules we need.
#

rewards = []
rewards_eval = []

# Main loop

collected_frames = 0
pbar = tqdm.tqdm(total=total_frames)
r0 = None
for i, tensordict in enumerate(collector):

    # update weights of the inference policy
    collector.update_policy_weights_()

    if r0 is None:
        r0 = tensordict["next", "reward"].mean().item()
    pbar.update(tensordict.numel())

    # extend the replay buffer with the new data
    current_frames = tensordict.numel()
    collected_frames += current_frames
    replay_buffer.extend(tensordict.cpu())

    # optimization steps
    if collected_frames >= init_random_frames:
        for _ in range(update_to_data):
            # sample from replay buffer
            sampled_tensordict = replay_buffer.sample().to(device)

            # Compute loss
            loss_dict = loss_module(sampled_tensordict)

            # optimize
            loss_dict["loss_actor"].backward()
            gn1 = torch.nn.utils.clip_grad_norm_(
                loss_module.actor_network_params.values(True, True), 10.0
            )
            optimizer_actor.step()
            optimizer_actor.zero_grad()

            loss_dict["loss_value"].backward()
            gn2 = torch.nn.utils.clip_grad_norm_(
                loss_module.value_network_params.values(True, True), 10.0
            )
            optimizer_value.step()
            optimizer_value.zero_grad()

            gn = (gn1**2 + gn2**2) ** 0.5

            # update priority
            if prb:
                replay_buffer.update_tensordict_priority(sampled_tensordict)
            # update target network
            target_net_updater.step()

    rewards.append(
        (
            i,
            tensordict["next", "reward"].mean().item(),
        )
    )
    td_record = recorder(None)
    if td_record is not None:
        rewards_eval.append((i, td_record["r_evaluation"].item()))
    if len(rewards_eval) and collected_frames >= init_random_frames:
        target_value = loss_dict["target_value"].item()
        loss_value = loss_dict["loss_value"].item()
        loss_actor = loss_dict["loss_actor"].item()
        rn = sampled_tensordict["next", "reward"].mean().item()
        rs = sampled_tensordict["next", "reward"].std().item()
        pbar.set_description(
            f"reward: {rewards[-1][1]: 4.2f} (r0 = {r0: 4.2f}), "
            f"reward eval: reward: {rewards_eval[-1][1]: 4.2f}, "
            f"reward normalized={rn :4.2f}/{rs :4.2f}, "
            f"grad norm={gn: 4.2f}, "
            f"loss_value={loss_value: 4.2f}, "
            f"loss_actor={loss_actor: 4.2f}, "
            f"target value: {target_value: 4.2f}"
        )

    # update the exploration strategy
    actor_model_explore[1].step(current_frames)

collector.shutdown()
del collector

###############################################################################
# Experiment results
# ------------------
#
# We make a simple plot of the average rewards during training. We can observe
# that our policy learned quite well to solve the task.
#
# .. note::
#   As already mentioned above, to get a more reasonable performance,
#   use a greater value for ``total_frames`` for example, 1M.

from matplotlib import pyplot as plt

plt.figure()
plt.plot(*zip(*rewards), label="training")
plt.plot(*zip(*rewards_eval), label="eval")
plt.legend()
plt.xlabel("iter")
plt.ylabel("reward")
plt.tight_layout()

###############################################################################
# Conclusion
# ----------
#
# In this tutorial, we have learned how to code a loss module in TorchRL given
# the concrete example of DDPG.
#
# The key takeaways are:
#
# - How to use the :class:`~torchrl.objectives.LossModule` class to code up a new
#   loss component;
# - How to use (or not) a target network, and how to update its parameters;
# - How to create an optimizer associated with a loss module.
#
# Next Steps
# ----------
# 
# To iterate further on this loss module we might consider:
# 
# - Using `@dispatch` (see `[Feature] Distpatch IQL loss module <https://github.com/pytorch/rl/pull/1230>`_.)
# - Allowing flexible TensorDict keys.
#