Error in backpropagation

[SS8188]

In multi-class classification exercise(spiral dataset)
When I run the training loop,it says:
RuntimeError: Trying to backward through the graph a second time
I have not used train_loss.backward() anywhere else in the notebook.
I tried specifying retain_graph=True as an argument to the backward pass but it is not resolving the error.
I tried changing the LR & random_seed but it is of no help.
I think,all the data is of the correct dtype with requires_grad=True and on the same device as model so please suggest something new.
notebook link:https://colab.research.google.com/drive/1FWk5PTsN5KSl9HSFWu_OkGZ9eUSZgqhf#scrollTo=LyJ7SnsDNaU7&uniqifier=1