Add line by line option to mlm/plm scripts (#8240)
* Make line by line optional in run_mlm
* Add option to disable dynamic padding
* Add option to plm too and update README
* Typos
* More typos
* Even more typos
* Apply suggestions from code review
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
