Created by: xiaolil1

This PR is only for reference, no need to run pre-ci and merge.

To make INT8 kernel more readable, i make a temp file named "fc_mkldnn_int8_op.cc", it will be merged and align to MKLDNN FP32 kernel "fc_mkldnn_op.cc" coding style as well as adding UT if there is PR merge requirement in the future.