https://zhuanlan.zhihu.com/p/40920384
https://zhuanlan.zhihu.com/p/31547842
http://www.wildml.com/2016/01/attention-and-memory-in-deep-learning-and-nlp/
https://mp.weixin.qq.com/s/5D8ZjMjX7T-sHrdWIODQMA
https://zhuanlan.zhihu.com/p/27769667
https://github.com/tensorflow/nmt
https://github.com/princewen/tensorflow_practice/tree/master/nlp/chat_bot_seq2seq_attention