[FSDP][optim_state_dict][2/N] Add _get_fqn_to_fsdp_param_info to map from original FQN to flat_param (#89899)
**Motivation:**
Add a helper to map from the FQN to the corresponding flat_param. The helper will directly get flat_param from fsdp_state and flat_handler as flat_param is not registered to the module if `use_orig_params` is True.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89899
Approved by: https://github.com/awgu
