Remove improper src_mask from encoder tutorial

The tutorial is using a transformer encoder and the mask used was for
masking a decoder which is not part of the tutorial.

The mask is removed. Some variable names are changed to better reflect
the purpose of the variable. Also, some unused imports are removed.

Fixes #1877

Signed-off-by: BJ Hargrave <hargrave@us.ibm.com>

Author: bjhargrave

beginner_source/transformer_tutorial.py

@@ -36,12 +36,8 @@
 # of the word (see the next paragraph for more details). The
 # ``nn.TransformerEncoder`` consists of multiple layers of
 # `nn.TransformerEncoderLayer <https://pytorch.org/docs/stable/generated/torch.nn.TransformerEncoderLayer.html>`__.
-# Along with the input sequence, a square attention mask is required because the
-# self-attention layers in ``nn.TransformerDecoder`` are only allowed to attend
-# the earlier positions in the sequence. For the language modeling task, any
-# tokens on the future positions should be masked. To produce a probability
-# distribution over output words, the output of the ``nn.TransformerEncoder``
-# model is passed through a linear layer followed by a log-softmax function.
+# To produce a probability distribution over output words, the output of 
+# the ``nn.TransformerEncoder`` model is passed through a linear layer.
 #
 
 import math
@@ -51,7 +47,6 @@
 
 import torch
 from torch import nn, Tensor
-import torch.nn.functional as F
 from torch.nn import TransformerEncoder, TransformerEncoderLayer
 from torch.utils.data import dataset
 
@@ -64,19 +59,19 @@ def __init__(self, ntoken: int, d_model: int, nhead: int, d_hid: int,
         self.pos_encoder = PositionalEncoding(d_model, dropout)
         encoder_layers = TransformerEncoderLayer(d_model, nhead, d_hid, dropout)
         self.transformer_encoder = TransformerEncoder(encoder_layers, nlayers)
-        self.encoder = nn.Embedding(ntoken, d_model)
+        self.embedding = nn.Embedding(ntoken, d_model)
         self.d_model = d_model
-        self.decoder = nn.Linear(d_model, ntoken)
+        self.linear = nn.Linear(d_model, ntoken)
 
         self.init_weights()
 
     def init_weights(self) -> None:
         initrange = 0.1
-        self.encoder.weight.data.uniform_(-initrange, initrange)
-        self.decoder.bias.data.zero_()
-        self.decoder.weight.data.uniform_(-initrange, initrange)
+        self.embedding.weight.data.uniform_(-initrange, initrange)
+        self.linear.bias.data.zero_()
+        self.linear.weight.data.uniform_(-initrange, initrange)
 
-    def forward(self, src: Tensor, src_mask: Tensor) -> Tensor:
+    def forward(self, src: Tensor, src_mask: Tensor = None) -> Tensor:
         """
         Arguments:
             src: Tensor, shape ``[seq_len, batch_size]``
@@ -85,18 +80,13 @@ def forward(self, src: Tensor, src_mask: Tensor) -> Tensor:
         Returns:
             output Tensor of shape ``[seq_len, batch_size, ntoken]``
         """
-        src = self.encoder(src) * math.sqrt(self.d_model)
+        src = self.embedding(src) * math.sqrt(self.d_model)
         src = self.pos_encoder(src)
         output = self.transformer_encoder(src, src_mask)
-        output = self.decoder(output)
+        output = self.linear(output)
         return output
 
 
-def generate_square_subsequent_mask(sz: int) -> Tensor:
-    """Generates an upper-triangular matrix of ``-inf``, with zeros on ``diag``."""
-    return torch.triu(torch.ones(sz, sz) * float('-inf'), diagonal=1)
-
-
 ######################################################################
 # ``PositionalEncoding`` module injects some information about the
 # relative or absolute position of the tokens in the sequence. The
@@ -286,7 +276,6 @@ def get_batch(source: Tensor, i: int) -> Tuple[Tensor, Tensor]:
 # to prevent gradients from exploding.
 #
 
-import copy
 import time
 
 criterion = nn.CrossEntropyLoss()
@@ -299,16 +288,13 @@ def train(model: nn.Module) -> None:
     total_loss = 0.
     log_interval = 200
     start_time = time.time()
-    src_mask = generate_square_subsequent_mask(bptt).to(device)
 
     num_batches = len(train_data) // bptt
     for batch, i in enumerate(range(0, train_data.size(0) - 1, bptt)):
         data, targets = get_batch(train_data, i)
-        seq_len = data.size(0)
-        if seq_len != bptt:  # only on last batch
-            src_mask = src_mask[:seq_len, :seq_len]
-        output = model(data, src_mask)
-        loss = criterion(output.view(-1, ntokens), targets)
+        output = model(data)
+        output_flat = output.view(-1, ntokens)
+        loss = criterion(output_flat, targets)
 
         optimizer.zero_grad()
         loss.backward()
@@ -330,14 +316,11 @@ def train(model: nn.Module) -> None:
 def evaluate(model: nn.Module, eval_data: Tensor) -> float:
     model.eval()  # turn on evaluation mode
     total_loss = 0.
-    src_mask = generate_square_subsequent_mask(bptt).to(device)
     with torch.no_grad():
         for i in range(0, eval_data.size(0) - 1, bptt):
             data, targets = get_batch(eval_data, i)
             seq_len = data.size(0)
-            if seq_len != bptt:
-                src_mask = src_mask[:seq_len, :seq_len]
-            output = model(data, src_mask)
+            output = model(data)
             output_flat = output.view(-1, ntokens)
             total_loss += seq_len * criterion(output_flat, targets).item()
     return total_loss / (len(eval_data) - 1)