Refactor column mappings for question answering datasets
_The documentation is not available anymore as the PR was closed or merged._ > Thanks.
> 
> I have no visibility about this, but if you say it is more useful for AutoTrain this way...

Thanks for the review @albertvillanova ! Yes, I need some way to reconstruct the original column names with a period because that's how they appear after we flatten the nested columns. In any case, we can adjust this later if needed :) Does that mean that we need to change the metadata? > Does that mean that we need to change the metadata?

Yes, but this PR takes care of it :) Oh good! thanks for the heads up!
This PR tweaks the keys in the metadata that are used to define the column mapping for question answering datasets. This is needed in order to faithfully reconstruct column names like `answers.text` and `answers.answer_start` from the keys in AutoTrain.

As observed in https://github.com/huggingface/datasets/pull/4367 we cannot use periods `.` in the keys of the YAML tags, so a decision was made to use a flat mapping with underscores. For QA datasets, however, it's handy to be able to reconstruct the nesting -- hence this PR.

cc @sashavor 