[FSDP][optim_state_dict] Fix `_is_named_optimizer` when the state is empty (#93303)
Optimizer state is not eager initializaion -- only NamedOptimizer and KeyedOptimizer are. This PR makes it `_is_named_optimizer` work with regular optimizers.
Differential Revision: [D42858589](https://our.internmc.facebook.com/intern/diff/D42858589/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93303
Approved by: https://github.com/fduwjj
