[lint doc] how to fix flake errors if pre-commit hook wasn't there (#49345)
Summary:
This PR adds instructions on what to do if one committed into a PR branch w/o having a pre-commit hook enabled and having CI report flake8 errors.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49345
Reviewed By: cpuhrsch
Differential Revision: D25683167
Pulled By: soumith
fbshipit-source-id: 3c45c866e1636c116d2cacec438d62c860e6b854
