[FSDP] Remove unneeded checks (#83150)
@awgu pointed out these checks aren't really doing anything, as they just make sure we're setting training state in certain ways throughout FSDP and is sort of arbitrary. So, removing them to avoid confusion.
We still keep the checking around `_post_backward_called` because this is needed in `finalize_params` for now.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83150
Approved by: https://github.com/awgu
