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torchax

README.md

torchax: Running PyTorch on TPU

torchax! is a backend for PyTorch, allowing users to run PyTorch on Google CloudTPUs. torchax! is also a library for providing graph-level interoperability between PyTorch and Jax.

This means, with torchax you can:

  • Run PyTorch code on TPU with as little as 2 lines of code change.
  • Call a jax function from a pytorch function, passing in jax.Arrays
  • Call a pytorch function from a jax function, passing in a torch.Tensor subclass.
  • Use jax features such as jax.grad, optax and GSMPD to train a Pytorch model.
  • Use a Pytorch model as feature extractor and use it with a Jax model. etc etc.

Install

On Google Cloud TPU:

First install torch CPU:

pip install torch --index-url https://download.pytorch.org/whl/cpu

Then install jax TPU:

pip install -U jax[tpu]

Finally install torchax

pip install torchax

On GPU machines:

First install torch CPU:

pip install torch --index-url https://download.pytorch.org/whl/cpu

Then install jax CUDA:

pip install -U jax[cuda12]

Finally install torchax

pip install torchax

On CPU machines (mac included)

First install torch CPU:

# Linux
pip install torch --index-url https://download.pytorch.org/whl/cpu

# OR Mac:
pip install torch

Then install jax CPU:

pip install -U jax

Finally install torchax

pip install torchax

NOTE: if you like metal support for Apple devices then install the metal version of jax: https://developer.apple.com/metal/jax/

Installing torchax from source

Still need to install torch CPU and Jax of your accelerator (GPU, TPU or None).

pip install git+https://github.com/pytorch/xla.git#subdirectory=torchax

Run a model

Now let's execute a model under torchax. We'll start with a simple 2-layer model it can be in theory any instance of torch.nn.Module.

import torch
import torch.nn as nn
import torch.nn.functional as F


class MyModel(nn.Module):
    def __init__(self):
        super().__init__()
        self.fc1 = nn.Linear(28 * 28, 120)
        self.fc2 = nn.Linear(120, 84)
        self.fc3 = nn.Linear(84, 10)

    def forward(self, x):
        x = x.view(-1, 28 * 28)
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = self.fc3(x)
        return x

m = MyModel()

# Execute this model using torch
inputs = torch.randn(3, 3, 28, 28)
print(m(inputs))

This model m contains 2 parts: the weights that is stored inside of the model and it's submodules (nn.Linear).

To execute this model with torchax; we need to enable torchax to capture pytorch ops. To enable this, use:

import torchax
torchax.enable_globally()

Then, a jax device will be available to use

inputs = torch.randn(3, 3, 28, 28, device='jax')
m = MyModel()
res = m(inputs)
print(type(res))  # outputs torchax.tensor.Tensor

torchax.tensor.Tensor is a torch.Tensor subclass that holds a jax.Array. You can inspect that jax array with res.jax()

What is happening behind the scene:

We took the approach detailed in new device recipe by Alban (@albanD); using jax.Array for the raw_data.

In other words, When a torch op is executed inside of env context manager (which is enabled with torchax.enable_globally()), we can swap out the implementation of that op written in Jax.

When a model's constructor runs, it will call some tensor constructor, such as torch.rand, torch.ones or torch.zeros etc to create its weights. The constructor will create an torch.Tensor subclass that contains a jax.Array.

Then, each subsequent op can unpack the jax.Array, call the op implementation, and wraps it back into torch.Tensor subclass.

See more at how_it_works and ops registry.

Executing with jax.jit

The above script will execute the model using eager mode Jax as backend. This does allow executing torch models on TPU, but is often slower than what we can achieve with jax.jit.

jax.jit is a function that takes a Jax function (i.e. a function that takes jax array and returns jax array) into the same function, but faster.

We have made the jax_jit decorator that would accomplish the same with functions that takes and returns torch.Tensor. To use this, the first step is to create a functional version of this model: this means the parameters should be passed in as input instead of being attributes on class:

def model_func(param, inputs):
  return torch.func.functional_call(m, param, inputs)

Here we use torch.func.functional_call from PyTorch to replace the model weights with param, then call the model. This is roughly equivalent to:

def model_func(param, inputs):
  m.load_state_dict(param)
  return m(*inputs)

Now, we can apply jax_jit

from torchax.interop import jax_jit
model_func_jitted = jax_jit(model_func)
print(model_func_jitted(new_state_dict, inputs))

See more examples at eager_mode.py and the (examples folder)[examples/]

However, to ease the idiom of creating functional model and calling it with parameters, we also created the JittableModule helper class.

So the above can be written as:

from torchax.interop import JittableModule

m_jitted = JittableModule(m)
res = m_jitted(...)

The first time that m_jitted is called , it will trigger jax.jit then the subsequent computation with inputs of same shape will be fast.

Citation:

@software{torchax, author = {Han Qi, Chun-nien Chan, Will Cromar, Manfei Bai, Kevin Gleanson}, title = {torchax: PyTorch on TPU and Jax interoperability}, url = {https://github.com/pytorch/xla/tree/master/torchax} version = {0.0.4}, date = {2025-02-24}, }

Maintainers & Contributors:

This library is created and maintained by the PyTorch/XLA team at Google Cloud.

However, it benefitted from many direct and indirect contributions outside of the team. Many of them done by fellow Googlers using Google's 20% project policy, others by partner teams.

Here is the full list of contributors by 2025-02-25.

Han Qi (qihqi), Pytorch / XLA Manfei Bai (manfeibai), Pytorch / XLA Will Cromar (will-cromar), Meta Milad Mohammadi (miladm), Pytorch / XLA Siyuan Liu (lsy323), Pytorch / XLA Bhavya Bahl (bhavya01), Pytorch / XLA Pei Zhang (zpcore), Pytorch / XLA Yifei Teng (tengyifei), Pytorch / XLA Chunnien Chan (chunnienc), Google, ODML Alban Desmaison (albanD), Meta, Pytorch Simon Teo (simonteozw), Google(20%) David Huang (dvhg), Google(20%) Barni Seetharaman (barney-s), Google(20%) Anish Karthik (anishfish2) , Google(20%) Yao Gu (guyao) , Google(20%) Yenkai Wang (yenkwang) , Google(20%) Greg Shikhman (commander) , Google(20%) Matin Akhlaghinia (matinehAkhlaghinia), Google(20%) Tracy Chen (tracych477), Google(20%) Matthias Guenther (mrguenther) , Google(20%) WenXin Dong (wenxindongwork), Google(20%) Kevin Gleason (GleasonK) , Google, StableHLO Nupur Baghel (nupurbaghel), Google(20%) Gwen Mittertreiner (gmittert), Google(20%) Zeev Melumian (zmelumian), Lightricks Vyom Sharma (vyom1611), Google(20%) Shitong Wang (ShitongWang), Adobe Rémi Doreau (ayshiff), Google(20%) Lance Wang (wang2yn84), Google, CoreML Hossein Sarshar (hosseinsarshar) , Google(20%) Daniel Vega-Myhre (danielvegamyhre) , Google(20%) Tianqi Fan (tqfan28), Google(20%) Jim Lin (jimlinntu), Google(20%) Fanhai Lu (FanhaiLu1), Google Cloud DeWitt Clinton (dewitt), Google PyTorch Aman Gupta (aman2930) , Google(20%)