Hi team timm,

As the title suggests, this PR allows the ImageDataset's __getitem__ to return more than the (img, target) tuple.
This is necessary for rate-constrained optimization (fairness repair methods for example) but could also be interesting for any other application that requires more than the img, target tuple.

For now, I only implemented this for ImageDataset using the ReaderHfds (as it is my use-case) but I'd be happy to generalize it if you are interested.

Cheers,
Augustin

PS: here is a sample code that uses this feature

waterbirds = create_dataset("grodino/waterbirds",split="test",additional_features="place")

for image, target, group in DataLoader(waterbirds):
    ...