Extend int8 quantize op to take scale and zero point from input
Summary: Extend int8 quantize op to take scale and zero point from input to support int8 PTQ productization of online training models.
Test Plan: buck test caffe2/caffe2/quantization/server:quantize_dnnlowp_op_test
Reviewed By: csummersea
Differential Revision: D21939660
fbshipit-source-id: 7ce2fbf9cd8a990c270f2187a49b1578ce76bc37
