Add more FSDP unit tests to cover core logic, freezing weights and flatten parameter wrapper (#66904)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66904
Add more FSDP unit tests to cover core logic, freezing weights and flatten parameter wrappe, these unit tests are refactored to be aligned with PyTorch commonly used test classes
ghstack-source-id: 141335614
Test Plan: unit tests
Reviewed By: mrshenli
Differential Revision: D31779565
fbshipit-source-id: c727110d1d7570c0ec49e42cadfc9e9a5e440073
