Add torch/lib/protobuf to gitignore, fixes #18700 (#19019)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19019
ghimport-source-id: 84d36f8d27912d1d094d5672154b82187dd88761
Differential Revision: D14846615
Pulled By: ezyang
fbshipit-source-id: e402557ec321c85be3b28c8602b680246c8eecfe
