[caffe2] open source 2/4-bit SLS operators (#34903)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34903
Reattempt of D20461609
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: jianyuh
Differential Revision: D20495304
fbshipit-source-id: 66a99677583f50fd40e29c514710c7b1a8cdbc29
