ImageFolder improvements
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_3887). All of your documentation changes will be reflected on that endpoint.
This PR adds the following improvements to the `imagefolder` dataset:
* skip the extract step for image files (as discussed in https://github.com/huggingface/datasets/pull/2830#discussion_r816817919)
* option to drop labels by setting `drop_labels=True` (useful for image pretraining cc @NielsRogge). This is faster than loading a dataset and removing the `label` column because we don't need to iterate over the files to infer class labels.  