wrong return type for `IterableDataset.shard()`
Oops my bad ! thanks for reporting
### Describe the bug

`IterableDataset.shard()` has the wrong typing for its return as `"Dataset"`. It should be `"IterableDataset"`. Makes my IDE unhappy.

### Steps to reproduce the bug

look at [the source code](https://github.com/huggingface/datasets/blob/main/src/datasets/iterable_dataset.py#L2668)?

### Expected behavior

Correct return type as `"IterableDataset"`

### Environment info

datasets==3.1.0