[FSDP] Tighten post-bwd cast to `reduce_dtype` (#90615)
This lowers the `reduce_dtype` retrieval to the `handle` instead of the `state` in preparation for `fully_shard`, and this adds a guard to avoid a no-op `to()` call.
Note that this change pretty much gets overridden in following PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/90615
Approved by: https://github.com/rohan-varma
