Add per channel observer (#25887)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25887
ghstack-source-id: 90383258
Add per channel observer to compute the qparams for each channel.
Test Plan:
buck test mode/dev caffe2/test:quantization -- 'test_per_channel_minmax_observer'
buck test mode/dev caffe2/test:quantization -- 'test_per_channel_minmax_observer_scriptable'
Differential Revision: D17137226
fbshipit-source-id: 0b1c93e3cbcda86f5c4e30f7cd94c670f2665063
