Make IterableDataset.from_spark more efficient
@lhoestq would you be able to review this please and also approve the workflow? Sounds good to me :) feel free to run `make style` to apply code formatting The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5986). All of your documentation changes will be reflected on that endpoint.
Moved the code from using collect() to using toLocalIterator, which allows for prefetching partitions that will be selected next, thus allowing for better performance when iterating. 