[BE]: Add filelock typing to mypy stubs (#119390)
Realized we used filelock in some places, but didn't have a mypy type stub for it. Noticed it in this PR: https://github.com/pytorch/pytorch/pull/119386
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119390
Approved by: https://github.com/albanD, https://github.com/malfet
