Update chatbot_tutorial.py (#1212)

Author: albanD

beginner_source/chatbot_tutorial.py

@@ -968,9 +968,10 @@ def train(input_variable, lengths, target_variable, mask, max_target_len, encode
 
     # Set device options
     input_variable = input_variable.to(device)
-    lengths = lengths.to(device)
     target_variable = target_variable.to(device)
     mask = mask.to(device)
+    # Lengths for rnn packing should always be on the cpu
+    lengths = lengths.to("cpu")
 
     # Initialize variables
     loss = 0