Created by: Superjomn

First version of neural machine translation

• dynamic recurrent operator with beam search @Superjom doing
• fc @Xreki
• simple_gru @guoshengCS
• concat @Yancey1989
• first_seq and last_seq @luotao1 doing
• embedding (lookup ) @qingqing01 (dense table)
• simple_attention (maybe just a python function based on fc)
  • seq_softmax @Xreki
  • scale (element-wise mul op) @gongweibao
  • seq_expand @wanghaoshuang (doing, https://github.com/PaddlePaddle/Paddle/pull/4740)