Update chatbot_tutorial.py (#1535)

Fix lengths to cpu. See issue #1300

Co-authored-by: Brian Johnson <[email protected]>

Author: krkv

beginner_source/chatbot_tutorial.py

@@ -1207,7 +1207,7 @@ def evaluate(encoder, decoder, searcher, voc, sentence, max_length=MAX_LENGTH):
     input_batch = torch.LongTensor(indexes_batch).transpose(0, 1)
     # Use appropriate device
     input_batch = input_batch.to(device)
-    lengths = lengths.to(device)
+    lengths = lengths.to("cpu")
     # Decode sentence with searcher
     tokens, scores = searcher(input_batch, lengths, max_length)
     # indexes -> words