[FSDP][optim_state_dict][10/N] Make optim_state_dict and optim_state_dict_to_load public (#92118)
Make optim_state_dict and optim_state_dict_to_load public APIs and consolidate them with state_dict by using the same state_dict_type to decide how to perform the optimizer state_dict save and load.
Differential Revision: [D42488022](https://our.internmc.facebook.com/intern/diff/D42488022/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/92118
Approved by: https://github.com/rohan-varma
