retain undefined tensors in backward pass (#41490)
Summary:
Leave undefined tensors / None returned from custom backward functions as undefined/None instead of creating a tensor full of zeros. This change improves performance in some cases.
**This is BC-Breaking:** Custom backward functions that return None will now see it potentially being propagated all the way up to AccumulateGrad nodes. Potential impact is that .grad field of leaf tensors as well as the result of autograd.grad may be undefined/None where it used to be a tensor full of zeros. Also, autograd.grad may raise an error, if so, consider using allow_unused=True ([see doc](https://pytorch.org/docs/stable/autograd.html?highlight=autograd%20grad#torch.autograd.grad)) if it applies to your case.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41490
Reviewed By: albanD
Differential Revision: D22578241
Pulled By: heitorschueroff
fbshipit-source-id: f4966f4cb520069294f8c5c1691eeea799cc0abe
