Multi-image loading in Imagefolder dataset

### Feature request

Extend the `imagefolder` dataloading script to support loading multiple images per dataset entry.

This only really makes sense if a metadata file is present.

Currently you can use the following format (example `metadata.jsonl`:
```
{'file_name': 'path_to_image.png', 'metadata': ...}
...
```
which will return a batch with key `image` and any other metadata.

I would propose extending `file_name` to also accept a list of files, which would return a batch with key `images` and any other metadata.

### Motivation

This is useful for example in segmentation tasks in computer vision models, or in text-to-image models that also accept conditioning signals such as another image, feature map, or similar. Currently if I want to do this, I would need to write a custom dataset, rather than just use `imagefolder`.

### Your contribution

Would be open to doing a PR, but also happy for someone else to take it as I am not familiar with the datasets library.