Add repeat method to datasets
@lhoestq does this look reasonable? updated and added test cases! The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7198). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. thanks for the fixes!
Following up on discussion in #6623 and #7198 I thought this would be pretty useful for my case so had a go at implementing.

My main motivation is to be able to call iterable_dataset.repeat(None).take(samples_per_epoch) to safely avoid timeout issues in a distributed training setting. This would provide a straightforward workaround for several open issues related to this situation:  https://github.com/huggingface/datasets/issues/6437, https://github.com/huggingface/datasets/issues/6594, https://github.com/huggingface/datasets/issues/6623, https://github.com/huggingface/datasets/issues/6719.

@lhoestq let me know if this looks on the right track!