Load cached dataset as iterable 
@lhoestq Could you please look into that and review? The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5893). All of your documentation changes will be reflected on that endpoint.
To be used to train models it allows to load an IterableDataset from the cached Arrow file.  
See https://github.com/huggingface/datasets/issues/5481