Selecting tasks using their superset

[hynky1999]

What does this implement/fix? Explain your changes.

• Implements https://github.com/huggingface/lighteval/issues/294

• Add support for using multiple task groups in single run or mixing the task groups with normal task definitions

  Comments:

• Some typing fixes to get less red lines
• Moved custom module handling into registry, imo it make logically sense
• I use _x to signify a private property. I know there is no such a thing in python, but it's a common way to signify that the variable shouldn't be modified outside a class code

[clefourrier]

You'll need to fix the tests first :)

[clefourrier]

@hynky1999 where are we wrt this PR? is it still active?

[hynky1999]

Yes, just have other things to focus on rn. Will try to update it by the of this week

[clefourrier]

No urgency, just going through to see what to close

[clefourrier]

I skimmed it, that looks good, merging. Thanks for the update!