[FSDP][optim_state_dict] Consolidate the arguments and logic of optim_state_dict and optim_state_dict_to_load (#96534)
Summary:
The current `optim_state_dict()` does not require users to call `optim.state_dict()` first while `optim_state_dict_to_load()` requires users to call `optim.load_state_dict()`. This PR make both APIs provide the option for users not having to call the extra API.
This PR also changes the arguments order of `optim_state_dict_to_load` which is a breaking change. So we should do this asap before the API is adopted in production cases.
Test Plan: CI
Differential Revision: D43925068
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96534
Approved by: https://github.com/rohan-varma
