py: Add more authorship metadata from model card

[mofosyne]

This is also a bit of refactoring, but also would like to give users the option of adding some authorship data via model card metadata rather than via metadata override. However since Huggingface doesn't have one to one mapping, will need to add different style of ways people may want to enter such metadata organically.

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[compilade]

The change seems good overall, but I did not test it yet.

If you've tested that it works correctly in all relevant cases, feel free to merge.

[mofosyne]

Had a final lookover and at least we still got the basic test cases testing that area so should be ok.

I did identify future room for improvement in that I can see people wanting to put more specific information besides an array of string IDs for base_model and datasets (e.g. name, org, version, url, etc...). But let's wait and see if people use it enough to complain about it. Since we can always add detection for dicts, when the demand arrives.