RNN, LSTM, GRU cells

项目需要，先简记cell，有时间再写具体改进原因

RNN cell

LSTM cell:

GRU cell:

reference:

1、https://towardsdatascience.com/animated-rnn-lstm-and-gru-ef124d06cf45#50f0

2、https://towardsdatascience.com/illustrated-guide-to-lstms-and-gru-s-a-step-by-step-explanation-44e9eb85bf21