GitLab

* 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>