print a warning if a type annotation prefix is invalid according to mypy (#20884)
Summary:
This PR adds a check that prints a warning if a type annotation prefix isn't what mypy expects.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20884
Differential Revision: D15511043
Pulled By: Krovatkin
fbshipit-source-id: 9038e074807832931faaa5f4e69628f94f51fd72
