Provide flake8 install instructions. (#18629)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18629
ghimport-source-id: 66a8871c56ffcfa7d4bfdf601e180fae99194e28
Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18629 Provide flake8 install instructions.**
* #18628 Delete duplicated technical content from contribution_guide.rst
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Differential Revision: D14701004
fbshipit-source-id: b64292f0ef01b7894cf6b9ff8d5fd9e921c8d162
