Created by: wojtuss

This is a request for comments.

There is a need for applying INT8 quantization strategy in PaddlePaddle models. This patch adds a mechanism for applying INT8 quantization strategy similar to MKL-DNN approach.

A use_int8 op attribute and int8_placement_pass in the simplest form (similar to the use_mkldnn attribute and mkldnn_placement_pass) are added. The pass will allow a user to choose which operators should be quantized and use INT8 kernels.

We envisage more passes for INT8 optimization that would utilize the use_int8 attribute.

test=develop