Unbearably slow sorting on big mapped datasets
Hi ! `shard` currently returns a slow dataset by default, with examples evenly distributed in the dataset.

You can get a fast dataset using `contiguous=True` (which should be the default imo):

```python
dataset = dataset.shard(10, 0, contiguous=True)
```

This way you don't need to flatten_indices() and sort should be fast as well @lhoestq 

> contiguous=True (which should be the default imo)

For `IterableDataset`, it's not possible to implement contiguous sharding without knowing the number of examples in advance, so setting the default value to `contiguous=True` would result in an inconsistency between `Dataset` and `IterableDataset` (when we add `IterableDataset.shard`) Actually sharded iterable datasets are made of sub iterables that generally yield contiguous data no ? So in a way it's possible to shard an iterable dataset contiguously.

If the dataset is made of one shard it's indeed not possible to shard it contiguously though > Actually sharded iterable datasets are made of sub iterables that generally yield contiguous data no ? So in a way it's possible to shard an iterable dataset contiguously.

But sharding an iterable dataset by sharding its `gen_kwargs` would still yield approximate shards(not equal to `Dataset.shard`), no?  Yes indeed ! I understand the issue doesn't exist with non-mapped datasets, but if flattening is so much more efficient than sorting the indices, that's an issue in itself.

There are plenty of issues people posted for which the root cause turns out to be the same. It seems like mapped datasets are terribly inefficient. I think I saw some issue like that somewhere (about the mapped datasets in general), but can't find it now.

Maybe indices should be flattened before any additional processing, then.
### Describe the bug

For me, with ~40k lines, sorting took 3.5 seconds on a flattened dataset (including the flatten operation) and 22.7 seconds on a mapped dataset (right after sharding), which is about x5 slowdown. Moreover, it seems like it slows down exponentially with bigger datasets (wasn't able to sort 700k lines at all, with flattening takes about a minute).

### Steps to reproduce the bug

```Python
from datasets import load_dataset
import time

dataset = load_dataset("xnli", "en", split="train")

dataset = dataset.shard(10, 0)

print(len(dataset))

t = time.time()

# dataset = dataset.flatten_indices()  # uncomment this line and it's fast

dataset = dataset.sort("label", reverse=True, load_from_cache_file=False)

print(f"finished in {time.time() - t:.4f} seconds")

```

### Expected behavior

Expect sorting to take the same or less time than flattening and then sorting.

### Environment info

- `datasets` version: 2.12.1.dev0 (same with 2.12.0 too)
- Platform: Windows-10-10.0.22621-SP0
- Python version: 3.10.10
- Huggingface_hub version: 0.14.1
- PyArrow version: 12.0.0
- Pandas version: 2.0.1