add git submodule troubleshoot to CONTRIBUTING.md (#51458)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/51355.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/51458
Reviewed By: janeyx99
Differential Revision: D26176233
Pulled By: walterddr
fbshipit-source-id: 758e4203e11c81489234bbca812d1a3738504148
Author
Rong Rong (AI Infra)
