* add datacollator and dataset for next sentence prediction task
* bug fix (numbers of special tokens & truncate sequences)
* bug fix (+ dict inputs support for data collator)
* add padding for nsp data collator; renamed cached files to avoid conflict.
* add test for nsp data collator
* Style
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Lysandre <lysandre.debut@reseau.eseo.fr>