Add with_split to DatasetDict.map
Can you check this out, @lhoestq? cc @lhoestq @albertvillanova  @lhoestq
 @lhoestq
 @lhoestq The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_7368). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update. @lhoestq  @lhoestq please... Thank you so much for reviewing this PR! Have a great weekend~
#7356