fix crossentropy size in chatbot to align mask

Author: aobo-y

beginner_source/chatbot_tutorial.py

@@ -890,7 +890,7 @@ def forward(self, input_step, last_hidden, encoder_outputs):
 
 def maskNLLLoss(inp, target, mask):
     nTotal = mask.sum()
-    crossEntropy = -torch.log(torch.gather(inp, 1, target.view(-1, 1)))
+    crossEntropy = -torch.log(torch.gather(inp, 1, target.view(-1, 1)).squeeze(1))
     loss = crossEntropy.masked_select(mask).mean()
     loss = loss.to(device)
     return loss, nTotal.item()