[FSDP()][8/N] Refactor limiter's `_FreeEventQueue` (#87922)
This PR is easy. It just moves `_FreeEventQueue` into its own file `_limiter_utils.py`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87922
Approved by: https://github.com/rohan-varma, https://github.com/mrshenli
