比较：
https://www.jianshu.com/p/3774d46b665e
https://blog.csdn.net/sinat_33741547/article/details/82821782
https://www.cnblogs.com/jins-note/p/9715610.html
双向GRU：
https://blog.csdn.net/dcrmg/article/details/79306402
