[FSDP][optim_state_dict][1/N] Restructure _optim_state_dict to prepare the support of use_orig_param (#89898)
**Motivation:**
Restructure some APIs in _optim_state_dict.py to allow better future extension, mostly for supporting use_orig_params. NO logic change in this PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89898
Approved by: https://github.com/awgu
