[functorch] add lintrunner support (pytorch/functorch#783)
* add lintrunner support
Black and flake8 for now + workflow in CI
Your codebase is not clean for either, so either fix em all or define
exclusions.
You can run lint on all files by doing
```
git grep -Il . | xargs lintrunner
```
You can autoformat all files by doing
```
git grep -Il . | xargs lintrunner f
```
* fix checkout step
* modify to only run on functorch dir
* fix random line
Co-authored-by: Horace He <chilli@fb.com>
