Default histogram observer (#26622)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26622
ghstack-source-id: 90897064
Test Plan: buck test caffe2/test:quantization -- --print-passing-details
Differential Revision: D17508787
fbshipit-source-id: ae733ab35ec9b0233264014b8054d4d870fb05e1
