[BUG] sync LightevalTaskConfig docstring with types/defaults

Describe the bug

The docstring in LightevalTaskConfig (link) lags the type hints:

evaluation_splits says list[str] but it's actually ListLike so it can be tuple. Also, the default of validation isn't documented
- Imo a default of validation should not be there, as most people just have train/test
few_shots_split says str, but it's actually optional with a default of None

There's more cases (e.g. generation_size) but I didn't state them here.

N/a

Up-to-date docstrings. For typed code, it's a best practice to not to restate types in the docstring for this very reason.

lighteval==0.9.2