Add documentation for nn.Hardsigmoid and nn.functional.hardsigmoid. (#38120)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/38120
Test Plan: build docs locally and attach a screenshot to this PR.
Differential Revision: D21477815
Pulled By: zou3519
fbshipit-source-id: 420bbcfcbd191d1a8e33cdf4a90c95bf00a5d226
