Support c10::optional<Tensor> in custom C++ autograd function. (#37700)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37700
Certain autograd functions can have optional Tensor arguments. For
this purpose it would be nice to support c10::optional<Tensor> as an argument
for C++ autograd functions.
I've added the appropriate overload to ExtractVariables to ensure this works.
For an example, you can look at D21272807 in terms of how this is used.
ghstack-source-id: 103541789
Test Plan: waitforbuildbot
Differential Revision: D21363491
fbshipit-source-id: 0c8665e9bfe279e6b9ab84a889524fea11fa971c
