[FSDP] Rename `dtype` to `buffer_name_to_dtype` (#87934)
This PR is easy and only a rename. `dtype` does not convey that it is actually a `Dict[str, torch.dtype]` (when not `None`).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87934
Approved by: https://github.com/mrshenli
