[FSDP] Test `use_orig_params=True`, `no_sync()`, mixed precision (#91193)
This makes some minor fixes to ensure that `use_orig_params=True`, `no_sync()`, and mixed precision work together for `FULL_SHARD`, `SHARD_GRAD_OP`, and `NO_SHARD`.
The added unit test only checks that dtypes are correct since for FP16, it is hard to test for numeric parity against a baseline.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91193
Approved by: https://github.com/zhaojuanmao
