[state_dict][5/N] Add submodules save and load support (#111110)
It is not easy for user to do submodules save and load (e.g., fine tuning) because FSDP requires to get the root module. This PR enables the support of submodule save and load.
Differential Revision: [D50209727](https://our.internmc.facebook.com/intern/diff/D50209727/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111110
Approved by: https://github.com/wz337
ghstack dependencies: #111106, #111107, #111275, #111109
