fix: NER export

[JulesBelveze]

This PR aims at fixing the behaviour of the .export method of the CSVDataset and the ConllDataset for the NER task.

Checklist:

• I've added Google style docstrings to my code.
• I've used pydantic for typing when/where necessary.
• I have linted my code
• I have added tests to cover my changes.

[JulesBelveze]

@chakravarthik27 @ArshaanNazir the previous behaviour of the export method was quite weird:

• for Conll: only writing the entities with a label != 0 to file
• for CSV: one row was equal to one token

Hope I didn't screw up other components, but it didn't seem to be coupled to other objects.

[ArshaanNazir]

ConllDataset

Looks fine. I don't think it will affect other things in our flow.