Default observer and fake-quant for backends (#26627)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26627
ghstack-source-id: 91008337
Test Plan: buck test caffe2/test:quantization -- --print-passing-details
Differential Revision: D17518194
fbshipit-source-id: 1eb8a7a85dc811c4ee5228d68563abb157613ceb
