Initial support for retrieval models
This pull request provides intial support for incorporating retrieval models by 

- creating a new `RetrievalTranslationDataset` 
- adding support for training with nearest neighbors by providing a file with indices to them and relevant src and tgt files from where those sentences are to be retrieved
- adding a new inference script `nmt_transformer_infer_retrieval.py` which adds nearest neighbor to the sentences at test time