Fix (ex/llm): Fix integration with Lighteval Python API

[pablomlago]

Reason for this PR

Refactored integration with lighteval Python API to not avoid relying on the custom fork (see huggingface/lighteval#489 for extra context).

Changes Made in this PR

Refactored the lighteval evaluation code out of the LLM entrypoint, using a similar structure to https://github.com/huggingface/lighteval/blob/main/src/lighteval/main_accelerate.py.

Testing Summary

Added a new end-to-end test in test_llm (test_few_shot_eval) to evaluate a quantized model in lighteval.

Risk Highlight

• This PR includes code from another work (please detail).
• This PR contains API-breaking changes.
• This PR depends on work in another PR (please provide links/details).
• This PR introduces new dependencies (please detail).
• There are coverage gaps not covered by tests.
• Documentation updates required in subsequent PR.

Checklist

• Code comments added to any hard-to-understand areas, if applicable.
• Changes generate no new warnings.
• Updated any relevant tests, if applicable.
• No conflicts with destination dev branch.
• I reviewed my own code changes.
• Initial CI/CD passing.
• 1+ reviews given, and any review issues addressed and approved.
• Post-review full CI/CD passing.