GitLab

* [feat] FSDP: supporting multiple flatten parameter groups

- step 2: extending FPW to support multiple flat params groups
- FSDP still only use one group
- unit test does this the new code paths
- updated the changelog

* first cut, mypy passed

* test_flatten_params_wrapper.py::TestFlattenParams tests pass

* added two more test cases and fixed a case in the code

* fixed one bug with param_path_infos

* fixed two more tests with hardcoded flat_param names

* Update CHANGELOG.md
Co-authored-by: Min Xu <min.xu.public@gmail.com>