make flake8 failure blocking (#15675)
Summary:
Right now it just prints whatever flake8 errors and moves forward with the commit. This is too easy to miss.
It should block the commit so that the user can fix the issue
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15675
Differential Revision: D13567821
Pulled By: suo
fbshipit-source-id: 5f0de40ddd771bad8d6848417408cffbceb03183
