[Quant] Qadd: Add qint8 support backed by xnnpack (#73663)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73663
If built w/ USE_XNNPACK, for qint8 dtype, qadd is performed using xnnpack.
Reviewed By: kimishpatel
Differential Revision: D34470378
fbshipit-source-id: c09de117e26e2007c02d02711ca2ab53d2779b1d
(cherry picked from commit 7aa367a41bdc0b5f750d8da5b400e6fd62ec7719)
