Re-organize folder structure and hierarchy for DS2.

Reduce the config parsing codes (-360 Lines) for DS2 and make it looks cleaner.

rename mse into square_error after dev branch update.

Revert back to support input-hidden weights sharing between bi-directional RNNs.

1. Add options to enable and disable RNN weights sharing.
2. Set rnn_layer_size to 2048 by default.
3. Revert back the striding steps of 1st conv layer from 2 to 3.
4. Revert back to BRelu.

Above follows DS2 papers.

Fix a bug in use_gru argument parsing.

Print log to pfs for cloud training and set use_gru to False by default.

Add GRU support for DS2 model.

Rename self.local_data to self._local_data in class DataGenerator.

Fixed a serious error of bidirectional simple rnn for DS2.

metric ndcg with power 2

Fix a typo caused exception for audio_featurizer.py.

Fix dssm infer error with gru