Fix model to be properly exported to ONNX (#1144)

Co-authored-by: Brian Johnson <brianjo@fb.com>

Author: Thiago Crepaldi

beginner_source/transformer_tutorial.py

@@ -77,9 +77,9 @@ def init_weights(self):
         self.decoder.weight.data.uniform_(-initrange, initrange)
 
     def forward(self, src):
-        if self.src_mask is None or self.src_mask.size(0) != len(src):
+        if self.src_mask is None or self.src_mask.size(0) != src.size(0):
             device = src.device
-            mask = self._generate_square_subsequent_mask(len(src)).to(device)
+            mask = self._generate_square_subsequent_mask(src.size(0)).to(device)
             self.src_mask = mask
 
         src = self.encoder(src) * math.sqrt(self.ninp)