NLP: Seq2seq, LuongAttention, SimpleRNN, GRURNN, LSTMRNN

[JingqingZ]

Checklist

• I've tested that my changes are compatible with the latest version of Tensorflow.
• I've read the Contribution Guidelines
• I've updated the documentation if necessary.

Motivation and Context

This PR includes updates on the followings:

• Seq2seq model, Seq2seq model with Luong attention
• Modified RNN layers including SimpleRNN, GRURNN, LSTMRNN

• The latest layers and models are tested on synthetic data.

Description

[zsdonghao]

Good job. Could you also help to update the Chinese docs here? https://github.com/tensorlayer/tensorlayer-chinese/tree/master/docs/modules

Best