Implement RNN model

[swangcs]

Please study the RNN model and its important variations (e.g. attention mechanism, transformer) carefully, then using your own idea (not replicate existing paper) to implement it for our bus arrival time prediction problem.

[Ruixinhua]

The code of implementation and test of lstm-rnn and other models are updated in commit 060fba7.
The performance and efficiency of lstm are not good at all, and I guess the implementation is incorrect. Maybe we should input variable-length sequences when training the model. Another problem when evaluating the model is that the value of MAPE is much lower compared to the 2014 paper and 2018 paper, and this same problem happened when tested dataset of 2014 paper. For more details, check code in model_evaluation/bus_evaluation.py