Add IterableDataset `__repr__`
Example for glue sst2:

Dataset

```
DatasetDict({
    test: Dataset({
        features: ['sentence', 'label', 'idx'],
        num_rows: 1821
    })
    train: Dataset({
        features: ['sentence', 'label', 'idx'],
        num_rows: 67349
    })
    validation: Dataset({
        features: ['sentence', 'label', 'idx'],
        num_rows: 872
    })
})
```

IterableDataset (new)

```
IterableDatasetDict({
    test: IterableDataset({
        features: ['sentence', 'label', 'idx'],
        n_shards: 1
    })
    train: IterableDataset({
        features: ['sentence', 'label', 'idx'],
        n_shards: 1
    })
    validation: IterableDataset({
        features: ['sentence', 'label', 'idx'],
        n_shards: 1
    })
})
```

IterableDataset (before)

```
{'test': <datasets.iterable_dataset.IterableDataset object at 0x130d421f0>, 'train': <datasets.iterable_dataset.IterableDataset object at 0x136f3aaf0>, 'validation': <datasets.iterable_dataset.IterableDataset object at 0x136f4b100>}
{'sentence': 'hide new secretions from the parental units ', 'label': 0, 'idx': 0}
```