[FSDP][1/N] Split `fully_shard` unit tests (#92296)
This PR splits `test_fully_shard.py` into `fully_shard/test_fully_shard<...>.py`. This should help improve readability and avoid some future rebase conflicts.
The only other real change is resolving a `TODO` for using `run_subtests` in the model checkpointing unit tests.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92296
Approved by: https://github.com/mrshenli
