Tidy up dependency groups

[lewtun]

This PR reorganises the dependency groups so that things like ruff and pytest are not part of the core library. This is how we do it in the other HF libs and is important because when these utilities are pinned to specific versions, it is not simple to integrate lighteval within another codebase / environment where the versions can clash.

One added bonus, is that one can now just define:

pip install -e '.[dev]'

to get the core set of dependencies during development :)

[clefourrier]

LGTM