Fixing the data collator

[EmYassir]

Changelogs

• enumerate the changes of that PR.
  Fixed the data collator.
  #fix

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Checklist:

• Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted. Eventually consider making a new tutorial for new features.
• Write concise and explanatory changelogs below.
• If possible, assign one of the following labels to the PR: feature, fix or test (or ask a maintainer to do it for you).

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discussion related to that PR

[maclandrol]

pad will fail if you are not using a copy of samples on which you pop the unused key.

It's designed to either tokenize on the fly or pad a pretokenized data. If the data is pretokenized your changes will raise an error.