Extend int8 FC op to take scale and zero point from input
Summary: Extend int8 FC 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:fully_connected_dnnlowp_op_test
Reviewed By: csummersea
Differential Revision: D21944884
fbshipit-source-id: 2094827da903f3993afe4f8cf6e70286b195321d
