Created by: kexinzhao

fix #8174 (closed) fix #8170 (closed) fix #8161 (closed) fix #8062 (closed) fix #8059 (closed)

The emphasize of the pr is to make the pruning method and save/load_inference_model to work with RNN related program desc containing for example while operator.

Since other RNN based book chapters like machine translation depends on this PR. We'd better review and merge it quickly.

Minor issues like this can be fixed in other PRs. Also the C++ inference example for this chapter will probably also be added in future PRs.