Integrated GRU implementation into C2
Integrated GRU implementation into C2

Summary:
Fixed unit test failures for GRU cell first implemented in D5778202

- GRUCell implementation added to rnn_cell.py
- GRU with recurrent attention test added to seq2seq_model_caffe2.py
- seq2seq_rnn.py
    - Added specific behavior for 'gru' cell type
        - in LSTMWithAttentionDecoder, output_indices fix for GRU cells
        - in build_initial_rnn_decoder_states, don't process cell state for GRU cells

Reviewed By: salexspb

Differential Revision: D6316441

fbshipit-source-id: 18668f3db62245c5cdaf3bfa473a40e0feba0473

davinwang/caffe2,davinwang/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,xzturn/caffe2,pietern/caffe2,Yangqing/caffe2,xzturn/caffe2,sf-wind/caffe2,Yangqing/caffe2,xzturn/caffe2,davinwang/caffe2,sf-wind/caffe2,Yangqing/caffe2,davinwang/caffe2,sf-wind/caffe2,caffe2/caffe2,sf-wind/caffe2,davinwang/caffe2,pietern/caffe2,Yangqing/caffe2,sf-wind/caffe2,pietern/caffe2,xzturn/caffe2,pietern/caffe2