add tests for ddp with activation check points (#52894)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52894
add two success cases and two failure cases for ddp with activation check points when grad_as_bucket_view = true and false
Test Plan: unit tests
Reviewed By: rohan-varma
Differential Revision: D26679895
fbshipit-source-id: a6f6cb22b4903ed8b1f7b8ed4fe8b13e102d8c21
