Added vocab script necessary for transformer xl

Added comment on register_buffer

Added data download script and note

Added transformer XL tutorial

Fixed tuple argument for LSTM tutorial

Adding utility functions for cleaning tutorial notebooks

ENH: Updated requirements.txt

Update README.md

NEW: Added BERT with fastai tutorial

BUG: Fixed error in transformer from scratch notebook

Added LSTM from scratch notebook

Updated readme

Added transformer from scratch notebook

Updated allennlp tutorials with predictors

BUG: Fixed bugs in ELMo/BERT vocab handling