GitLab

* fix saving and loading checkpoints with use_sharded_state=True

* mypy fix

* better fix of the infinite recursion

- we need to specifically call FSDP.state_dict from its local state_dict
- added unit test that fails without the fix and works with the fix
- fixed mypy for the overloaded functions

* make cpu-only fsdp work for state_dict at least
Co-authored-by: Min Xu <min.xu@acm.org>
Co-authored-by: Min Xu <min.xu.public@gmail.com>
Co-authored-by: Min Xu <m1n@fb.com>