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