[FSDP][state_dict][2/N] Move state_dict related enums/dataclasses/states to state_dict_utils.py, api.py and init_state_dict() (#88481)
**Motivation**:
Several Enums, Dataclasses and states defined in fully_sharded_data_paralle.py should be moved to a place where the composable FSDP can access. This PR does the move.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88481
Approved by: https://github.com/rohan-varma, https://github.com/awgu
