Update sparse.rst to warn about _values() (#71088)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/70357
Pull Request resolved: https://github.com/pytorch/pytorch/pull/71088
Reviewed By: jbschlosser
Differential Revision: D33511207
Pulled By: cpuhrsch
fbshipit-source-id: 9d0c5445842ed96999eb88445cbea7ae284b1a6f
