[FSDP] Add `record_function` for explicit prefetching (#105985)
Example:
<img width="568" alt="Screenshot 2023-07-25 at 7 41 43 PM" src="https://github.com/pytorch/pytorch/assets/31054793/5f3f07b3-97f4-4493-9cab-5619484e2f6d">
This can be particularly help when `with_stack=False`, in which case it is harder to tell the prefetch.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/105985
Approved by: https://github.com/fegin
