Add histogram observer (#23959)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23959
Add histogram observer that records the running histogram of tensor values along with min/max values.
ghstack-source-id: 90076996
Test Plan:
Added a test test_histogram_observer
buck test mode/dev caffe2/test:quantization -- 'test_histogram_observer'
buck test mode/dev caffe2/test:quantization -- 'test_observer_scriptable'
Differential Revision: D16692835
fbshipit-source-id: 0f047d3349cb9770fad4a2b6cb346c51d9e99cd4
