replace decoder with self.decoder in PyTorch Chatbot example. (#3090)

Co-authored-by: Svetlana Karslioglu <[email protected]>

beginner_source/chatbot_tutorial.py

@@ -1128,7 +1128,7 @@ def forward(self, input_seq, input_length, max_length):
         # Forward input through encoder model
         encoder_outputs, encoder_hidden = self.encoder(input_seq, input_length)
         # Prepare encoder's final hidden layer to be first hidden input to the decoder
-        decoder_hidden = encoder_hidden[:decoder.n_layers]
+        decoder_hidden = encoder_hidden[:self.decoder.n_layers]
         # Initialize decoder input with SOS_token
         decoder_input = torch.ones(1, 1, device=device, dtype=torch.long) * SOS_token
         # Initialize tensors to append decoded words to