Add an additional "Note" (#1000)

Add information about potential pitfall of not serializing the model state and keeping it as a reference during training.

Co-authored-by: holly1238 <[email protected]>

Author: tastyminerals

beginner_source/saving_loading_models.py

@@ -183,6 +183,14 @@
 #    ``load_state_dict()`` function. For example, you CANNOT load using
 #    ``model.load_state_dict(PATH)``.
 #
+# .. Note ::
+#    
+#    If you only plan to keep the best performing model (according to the 
+#    acquired validation loss), don't forget that ``best_model_state = model.state_dict()``
+#    returns a reference to the state and not its copy! You must serialize 
+#    ``best_model_state`` or use ``best_model_state = deepcopy(model.state_dict())`` otherwise
+#    your best ``best_model_state`` will keep getting updated by the subsequent training 
+#    iterations. As a result, the final model state will be the state of the overfitted model. 
 #
 # Save/Load Entire Model
 # ^^^^^^^^^^^^^^^^^^^^^^