[FSDP][optim_state_dict][7/N] Make FSDP support NamedOptimizer (#91160)
**What does this PR do?**
This PR refactors FSDP optimizer state_dict APIs to accept `NamedOptimizer` as the input optimizer. The key difference is that the state_dict returned by `NamedOptimizer` is already keyed as FQN. This PR majorly changes the internal mapping to allows the optimizer state_dict to be keyed as FQN.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91160
Approved by: https://github.com/fduwjj, https://github.com/rohan-varma
