Add note to docs of torch.unique (#29165)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/19151
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29165
Differential Revision: D18319890
Pulled By: soumith
fbshipit-source-id: 162afaecd5371446bec2a1769e0a8848ecffb002
