[FSDP] Add unittests to test distributed checkpointing
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77368
Add an unittest to test FSDP + torch.distributed._shard.checkpoint
Differential Revision: [D36175575](https://our.internmc.facebook.com/intern/diff/D36175575/)
Approved by: https://github.com/zhaojuanmao
