Tests for C++ custom autograd function API (#23628)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23628
More tests for autograd::Fuction based on python tests from test_autograd.py
Test Plan: Imported from OSS
Differential Revision: D16600992
fbshipit-source-id: 0cb8bfbcff315111dc4936e837ff859d0a1e251d
