[BUG] Unable to Download Dataset in Restricted Network Environment (No Access to Hugging Face or Mirrors)

## Describe the bug
In restricted or offline network environments, it is currently not possible to run `lighteval` because it attempts to download datasets (e.g., from the Hugging Face Hub or mirrors) without fallback options. When access to sites like `https://huggingface.co` or `https://hf-mirror.com` is blocked, dataset loading fails completely.

## To Reproduce
1. Run `lighteval` in an environment without internet access or with Hugging Face domains blocked.
2. Observe that dataset loading (e.g., via `load_dataset()` from the `datasets` library) fails.
3. Example error:

```
requests.exceptions.ConnectionError: HTTPSConnectionPool(host='huggingface.co', ...)
```

## Expected behavior
* Support for using pre-downloaded datasets from local disk (e.g., via environment variables or explicit local paths).
* Graceful handling of offline environments or clear documentation for air-gapped usage.

## Additional Context

* Many research or production environments have strict firewall policies or no internet access.
* This significantly limits the adoption of `lighteval` in such contexts.
* A partial workaround is to manually preload datasets into the Hugging Face `datasets` cache, but this is not natively supported by `lighteval`.

I have implemented support for running `lighteval` in fully offline mode and plan to submit a PR soon.
Hopefully, this will help address the issue and make the tool more accessible in restricted environments.