Add `expecttest` to CONTRIBUTING.md (#61163)
Summary:
Now expecttest is an independent library but `CONTRIBUTING.md` and `requirements.txt` do not mention the need of the library.
Related: https://github.com/pytorch/pytorch/pull/60658
Pull Request resolved: https://github.com/pytorch/pytorch/pull/61163
Reviewed By: heitorschueroff
Differential Revision: D29660296
Pulled By: ezyang
fbshipit-source-id: e2e86d42526c83bec7cdf7221e19fe83d9686103
