Enable more datasets in FDS

[adam-narozniak]

Relax the ValueError by changing to the warning.warn when the user wants to use the dataset that was not tested.

Changes

• Change the ValueError to warning.warn
• Adjust the tests to check if the code warns in case of untested datasets.

Motivation

By doing that, we allow

• many tabular datasets the library currently supports (but wouldn't if the ValueError was raised)
• vision datasets that have not been tested by us yet,
• potential contributions.

Questions

The way of keeping the reference to the tested datasets might be different that proposed by me - keep them in the list that corresponds to the categories in HF Hub.