Don't materialize output grads (#41821)
Summary:
Added a new option in AutogradContext to tell autograd to not materialize output grad tensors, that is, don't expand undefined/None tensors into tensors full of zeros before passing them as input to the backward function.
This PR is the second part that closes https://github.com/pytorch/pytorch/issues/41359. The first PR is https://github.com/pytorch/pytorch/pull/41490.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41821
Reviewed By: albanD
Differential Revision: D22693163
Pulled By: heitorschueroff
fbshipit-source-id: a8d060405a17ab1280a8506a06a2bbd85cb86461
