Custom op autograd tests (#30519)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30519
Re-enable them and write a few additional ones
ghstack-source-id: 95143051
Test Plan: unit tests
Differential Revision: D18729561
fbshipit-source-id: 8cefd8320913d72a450a3324bfd7c88faed072d7
