[caffe2] open source 2/4-bit SLS operators (#34783)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34783
Moving 2/4-bit SLS and row-wise 2/4-bit conversion operator to open source to be used by DLRM
Test Plan: CI
Reviewed By: yinghai
Differential Revision: D20461609
fbshipit-source-id: b3ef73ff10f2433afe06ffa73fe1145282d9ec4c
