[FSDP][Easy] Remove outdated TODO (#89217)
**Overview**
This PR removes an outdated TODO:
```
# TODO (awgu): When exposing the original parameters, we need to also
# use this attribute to prevent re-synchronizing parameters.
```
**Justification**
We only pass `managed_params` to `_sync_module_params_and_buffers()`, where `managed_params` is defined as
```
managed_params = list(_get_orig_params(root_module, state._ignored_params))
```
This `_get_orig_params()` call excludes parameters already flattened by FSDP. Thus, `_sync_module_params_and_buffers()` will not re-sync already-synchronized parameters. Each parameter appears in `managed_params` for some FSDP instance exactly once and hence is only synchronized once.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/89217
Approved by: https://github.com/mrshenli
