[autograd] fix allow_unused checking for C++ API (#34035)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34035
Bug for the conditon check in https://github.com/pytorch/pytorch/pull/24342, realized we don't have tests in either
python or cpp to catch this, so added testes for both python and cpp.
Thanks hczhu on capturing it!
Test Plan: Imported from OSS
Differential Revision: D20198837
Pulled By: wanchaol
fbshipit-source-id: 33846a14c0a8e7aac2e8328189d10c38a0d7e6ee
