Add xnnpack hardswish op (#56714)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56714
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55800
For mobile use xnnpack implementation of hardswish
Test Plan: buck test //xplat/caffe2:pt_xnnpack_test
Reviewed By: kimishpatel
Differential Revision: D27712306
fbshipit-source-id: c7f0b70482aeef2aaa1966e2c669f79ecd29caa7
