LSTM support

[SheilaWerth]

Hi all. I see that there is a LSTMPruner defined in function.py and that prune_lstm_in_channels and prune_lstm_out_channels aliases are defined. However, prune_lstm_in_channels and prune_lstm_out_channels are not included in the __all__ list that defines the public interface to the package. Why is that? Are there any shared examples of LSTM pruning? How do we recreate the results in the paper?