Exclude docs/cpp/src from flake8 (#48201)
Summary:
Currently when I run `flake8` locally I get [a bunch of extraneous warnings](https://pastebin.com/DMQevCtC) because the docs build puts a `pytorch-sphinx-theme` dir into `docs/cpp/src`. Those warnings don't show up in CI because the CI lint job doesn't generate that dir. This PR adds that to the Flake8 `exclude` list, similar to how `docs/src` is already present in that list.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48201
Reviewed By: walterddr, zhangguanheng66
Differential Revision: D25069130
Pulled By: samestep
fbshipit-source-id: 2fda9e813f54092398525b7fc97d0a8f7f835ca6
