Return a split Dataset in load_dataset
...instead of a DatasetDict.

```python
# now supported
ds = load_dataset("squad")
ds[0]  
for example in ds:
    pass

# still works
ds["train"]
ds["validation"]

# new
ds.splits  # Dict[str, Dataset] | None

# soon to be supported (not in this PR)
ds = load_dataset("dataset_with_no_splits")
ds[0]
for example in ds:
    pass
```

I implemented `Dataset.__getitem__` and `IterableDataset.__getitem__` to be able to get a split from a dataset.
The splits are defined by the `ds.info.splits` dictionary.

Therefore a dataset is a table that optionally has some splits defined in the dataset info. And a split dataset is the concatenation of all its splits.

I made as little breaking changes as possible. Notable breaking changes:
- `load_dataset("potato").keys() / .items() / .values() /` don't work anymore, since we don't return a dict
- same for `for split_name in load_dataset("potato")`, since we now iterate on the examples
- ..

TODO:
- [x] Update push_to_hub
- [x] Update save_to_disk/load_from_disk
- [ ] check for other breaking changes
- [ ] fix existing tests
- [ ] add new tests
- [ ] docs

This is related to https://github.com/huggingface/datasets/issues/5189, to extend `load_dataset` to return datasets without splits