Summary:
This is designed to make it easier to see how your codegen changes affected actual generated code.

Limitations:
A) This is NOT robust; if new directories are added that include generated files, they need to be added to tools/generated_dirs.txt.  Note that subdirectories of the list are not included.

B) This is particular to my workflow which I don't claim is generally applicable.  Ideally we would have a script that pumped out a diff that could be attached to PRs.

C) Only works on OSS and definitely won't work on windows.

How to use:
1) python setup.py ...
2) tools/git_add_generated_dirs
3) Edit codegen
4) python setup.py ...
4) git diff to see changes
5) If satisfied: tools/git_reset_generated_dirs, commit, etc.
   If not satisfied: Go to 3)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/12835

Reviewed By: ezyang

Differential Revision: D10452255

Pulled By: gchanan

fbshipit-source-id: 294fc74d41d1b840c7a26d20e05efd0aff154635