Add GRU in R

[ziyeqinghan]

I add GRU model and some reusable APIs which are similar to the APIs for LSTM in [#2329].

• mx.gru Training GRU Unrolled Model
• mx.gru.inference Create a GRU Inference Model
• mx.gru.forward Forward function for GRU inference model

[thirdwing]

LGTM.

BTW, don't forget the mid-term evaluation.

[ziyeqinghan]

Thank you for you reminder. I have completed the mid-term evaluation right now. :)

[thirdwing]

@ziyeqinghan Please rebase against the master. I will merge it.

[ziyeqinghan]

I have rebased it.