Refactor thai2transformers as utility package for transformers

[cstorm125]

transformers is currently the de facto way to train NLP models (maybe speech and image soon?). For Thai language, we have some difficulties using the default settings; for example, tokenization for sequence-based metrics such as BLEU is based on space tokenization. We also want to include some quality-of-life functions such as easily loading datasets into datasets objects and preprocessing functions that are available in the tutorial notebooks.

• Thai-language specific metrics
• Preprocessing functions
• Load datasets

PR #67