Enable loading data sets from files for custom tasks

[davebiagioni]

Purpose: Allow custom tasks to load datasets from local files, not only from the Hugging Face Hub. Useful for offline / air‑gapped / otherwise restricted environments

Changes

• Config: Add optional hf_data_files to LightevalTaskConfig.
• Loader: Forward hf_data_files as data_files to datasets.load_dataset in LightevalTask.download_dataset_worker.
• Examples: Add examples/custom_tasks_templates/custom_yourbench_task_from_files.py.
• Docs: Update docs/source/adding-a-custom-task.mdx with file-based usage.

Checklist

• Tests pass locally
• Pre-commit hooks pass locally
• Added/updated documentation

[HuggingFaceDocBuilderDev]

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[NathanHB]

Looking great ! few nits and good to merge if tests pass :)

[davebiagioni]

@NathanHB review comments addressed. thanks!