Deterministic split order in DatasetDict.map
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_3913). All of your documentation changes will be reflected on that endpoint. I'm surprised this is needed because the order of the `dict` keys is deterministic as of Python 3.6 (documented in 3.7). Is there a reproducer for this behavior? I wouldn't make this change unless it's absolutely needed because `sorted` modifies the initial order of the keys. Indeed this doesn't fix the issue apparently. Actually this is probably because the tokenizer used to process the second split is in a state that has been modified by the first split.

Therefore after reloading the first split from the cache, then the second split can't be reloaded since the tokenizer hasn't seen the first split (and therefore is considered a different tokenizer).
The order in which the splits are processed by `map` is not deterministic in `DatasetDict.map`. This can cause caching issues when the processing function is stateful and sensible to the order in which examples are processed

Close https://github.com/huggingface/datasets/issues/3847