参考：
https://zhuanlan.zhihu.com/p/28054589
http://pytorch123.com/FifthSection/Translation_S2S_Network/
https://www.zhihu.com/people/zhang-xu-99-99/posts
https://zhuanlan.zhihu.com/p/65591919
https://zhuanlan.zhihu.com/p/40920384
https://zhuanlan.zhihu.com/p/88376673