DOC: Add documentation for Tensor.is_nonzero (#37845)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/37438 by adding documentation for `Tensor.is_nonzero`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37845
Differential Revision: D21494422
Pulled By: mruberry
fbshipit-source-id: ee4f5979922d7c8100b5031d770ccdf59fe1c1a1
