Add environment variables used in CONTRIBUTING example (#22736)
Summary:
Some other environment variables can be added to speed things up for development.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/22736
Differential Revision: D16200904
Pulled By: soumith
fbshipit-source-id: 797ef91a863a244a6c96e0adf64d9f9b4c9a9582
