Dataset slice splits can't load training and validation at the same time
The expression "train+test" concatenates the splits.

The individual splits as separate datasets can be obtained as follows:
```python
train_ds, test_ds = load_dataset("<dataset_name>", split=["train", "test"])
train_10pct_ds, test_10pct_ds = load_dataset("<dataset_name>", split=["train[:10%]", "test[:%10]"])
```
### Describe the bug

According to the [documentation](https://huggingface.co/docs/datasets/v2.14.5/loading#slice-splits) is should be possible to run the following command:

`train_test_ds = datasets.load_dataset("bookcorpus", split="train+test")`

to load the train and test sets from the dataset. 

However executing the equivalent code:

`speech_commands_v1 = load_dataset("superb", "ks", split="train+test")`

only yields the following output:

> Dataset({
>     features: ['file', 'audio', 'label'],
>     num_rows: 54175
> })

Where loading the dataset without the split argument yields:

> DatasetDict({
>     train: Dataset({
>         features: ['file', 'audio', 'label'],
>         num_rows: 51094
>     })
>     validation: Dataset({
>         features: ['file', 'audio', 'label'],
>         num_rows: 6798
>     })
>     test: Dataset({
>         features: ['file', 'audio', 'label'],
>         num_rows: 3081
>     })
> })

Thus, the API seems to be broken in this regard. 

This is a bit annoying since I want to be able to use the split argument with `split="train[:10%]+test[:10%]"` to have smaller dataset to work with when validating my model is working correctly. 

### Steps to reproduce the bug

`speech_commands_v1 = load_dataset("superb", "ks", split="train+test")`

### Expected behavior

> DatasetDict({
>     train: Dataset({
>         features: ['file', 'audio', 'label'],
>         num_rows: 51094
>     })
>     test: Dataset({
>         features: ['file', 'audio', 'label'],
>         num_rows: 3081
>     })
> })

### Environment info

```
import datasets
print(datasets.__version__)
```
> 2.14.5

```
import sys
print(sys.version)
```
> 3.9.17 (main, Jul  5 2023, 20:41:20) 
> [GCC 11.2.0]