save_inference_model 如果fetch activation 的output，会丢掉 activation op
Created by: Superjomn
https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/fluid/framework/prune.cc#L93
Created by: Superjomn
https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/fluid/framework/prune.cc#L93