[FSDP][2/N] Move `fsdp_modules(root_only=False)` -> `_get_fsdp_states()` (#90861)
This PR migrates all internal usages of `FullyShardedDataParallel.fsdp_modules(root_only=False)` to `_get_fsdp_states()`. This is to unify the code paths for composable and wrapper FSDP.
This PR _does not_ change the usages in test files. This is because we should revisit those usages separately as a way to track which functionality for which we have not tested composable FSDP.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90861
Approved by: https://github.com/rohan-varma
