Add `torch.where` overload with only condition argument (#21986)
Summary:
Requested in https://github.com/pytorch/pytorch/issues/21798
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21986
Differential Revision: D16081577
Pulled By: zhangguanheng66
fbshipit-source-id: 658c0f451b833aceb1a41ee424c7990eec00bc02
