Created by: wojtuss

We add cpu_quantize_squash_pass, which squashes dequantize + quantize operator pairs in a model quantized using the cpu_quantize_pass (https://github.com/PaddlePaddle/Paddle/pull/16127). After the pass, dequantize + quantize pairs are removed or changed into requantize ops.

This pass works with the C-API quantization core (https://github.com/PaddlePaddle/Paddle/pull/15987) and cpu_quantize_pass (https://github.com/PaddlePaddle/Paddle/pull/16127), and is the second pass in a series of quantization passes.

The two passes are enough for quantization of ResNet50 and MobileNet-v1.