[FSDP][state_dict][3/N] Change how state_dict utils access attributes in _FSDPState (#88635)
**What This PR Does**
_state_dict_utils currently accesses the FSDP states through module. To enable composable FSDP state_dict, these accesses need to go through _FSDPState. module is still required for most APIs as state_dict has to access per-module information.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88635
Approved by: https://github.com/awgu
