question: RuntimeError: Trying to backward through the graph a second time
context: I'm migrating my repository to pytorch-lightning and I get the following error:
  RuntimeError: Trying to backward through the graph a second time, but the saved intermediate results have already been freed. Specify retain_graph=True when calling .backward() or autograd.grad() the first time.
  The CNNLSTM model seems to be the problem, what should I do?
  [My repository]
  Keiku/Action-Recognition-CNN-LSTM: Action recognition tutorial using UCF-101 dataset. https://github.com/Keiku/Action-Recognition-CNN-LSTM
answer: Hi @Keiku This error happens if you try to call backward on something twice in a row without calling optimizer.step. Are you able to share your LightningModule code? It looks like the code in your repo just uses vanilla pytorch.
  Thanks 😃