[FSDP] Add set_state_dict_type API to setup state_dict_type without using context manager (#86243)
FSDP.state_dict_type is a context manager. However, users may want to decide what state_dict is going to used during initialization. `set_state_dict_type` allows users to do so.
Differential Revision: [D40083670](https://our.internmc.facebook.com/intern/diff/D40083670/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/86243
Approved by: https://github.com/rohan-varma
