use flake8-mypy (#17721)
Summary:
Use flake8 installed with mypy checks so that our linter matches fbcode. Mypy type errors also provide valuable signal
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17721
Differential Revision: D14357778
Pulled By: eellison
fbshipit-source-id: d8c9ea3fe3b5f550c3b70fe259e0eabf95e4c92d
