[quant][refactor] Merge add and mul handler (#52651)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52651
Merging them for easier extensions to fp16 and more binary ops
Test Plan: Imported from OSS
Reviewed By: vkuzo
Differential Revision: D26600118
fbshipit-source-id: a1816e593cf3065afe87d2e6e44cdace13bf6aeb
