Int8 PTQ ops for online training (#39818)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39818
Add histogram collection and qparam update support for Int8 PTQ during online training
Add caffe2 wrappers for generating int8 quant params based on output activation samples from the LastNWindowCollector op.
Test Plan:
```
buck test mode/opt caffe2/caffe2/quantization/server:int8_gen_quant_params_test
```
Reviewed By: hx89
Differential Revision: D21984455
fbshipit-source-id: 9479c87a5b1867aec662ecd21fe7ad2bc7e8652c
