Completely synchronize behavior of Facebook flake8 and public flake8. (#18538)

Completely synchronize behavior of Facebook flake8 and public flake8. (#18538)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18538
ghimport-source-id: 665b09f158d1c5dd94686d4212792504b55b7f73

Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18538 Completely synchronize behavior of Facebook flake8 and public flake8.**

Previously, developers at Facebook had the very funny experience
wherein /usr/local/bin/flake8 behaved differently than a freshly
installed flake8 from pip.  In this commit, I add enough ignores to
.flake8 and install enough plugins to make the Facebook flake8
and public flake8 line up exactly.  These means you don't have
to care which flake8 you use; they all will report accurate information
on your Python files.

Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14652336

fbshipit-source-id: ba7776eaa139cf2e3df2e65349da6fd7c99acca4