Created by: guoshengCS
Add Transformer implementation. Add text.py to export:
'RNNCell', 'BasicLSTMCell', 'BasicGRUCell', 'RNN', 'DynamicDecode',
'BeamSearchDecoder', 'MultiHeadAttention', 'FFN',
'TransformerEncoderLayer', 'TransformerEncoder', 'TransformerDecoderLayer',
'TransformerDecoder', 'TransformerBeamSearchDecoder'
API的位置可以再看下