Add unit tests and more apis for text.py

[guoshengCS]

Add unit tests and more apis for text.py

Currently, text.py includes apis as follows:

__all__ = [
    'RNNCell',
    'BasicLSTMCell',
    'BasicGRUCell',
    'RNN',
    'BidirectionalRNN',
    'StackedRNNCell',
    'StackedLSTMCell',
    'LSTM',
    'BidirectionalLSTM',
    'StackedGRUCell',
    'GRU',
    'BidirectionalGRU',
    'DynamicDecode',
    'BeamSearchDecoder',
    'Conv1dPoolLayer',
    'CNNEncoder',
    'MultiHeadAttention',
    'FFN',
    'TransformerEncoderLayer',
    'TransformerEncoder',
    'TransformerDecoderLayer',
    'TransformerDecoder',
    'TransformerCell',
    'TransformerBeamSearchDecoder',
    'LinearChainCRF',
    'CRFDecoding',
    'SequenceTagging',
]