The order of the splits is not preserved
After investigation, I think the issue was introduced by the use of the Parquet export:
- #6448

I am proposing a fix.

CC: @lhoestq 
We had a regression and the order of the splits is not preserved. They are alphabetically sorted, instead of preserving original "train", "validation", "test" order.

Check: In branch "main"
```python
In [9]: dataset = load_dataset("adversarial_qa", '"adversarialQA")

In [10]: dataset
Out[10]: 
DatasetDict({
    test: Dataset({
        features: ['id', 'title', 'context', 'question', 'answers', 'metadata'],
        num_rows: 3000
    })
    train: Dataset({
        features: ['id', 'title', 'context', 'question', 'answers', 'metadata'],
        num_rows: 30000
    })
    validation: Dataset({
        features: ['id', 'title', 'context', 'question', 'answers', 'metadata'],
        num_rows: 3000
    })
})
```

Before (2.15.0) it was:
```python
DatasetDict({
    train: Dataset({
        features: ['id', 'title', 'context', 'question', 'answers', 'metadata'],
        num_rows: 30000
    })
    validation: Dataset({
        features: ['id', 'title', 'context', 'question', 'answers', 'metadata'],
        num_rows: 3000
    })
    test: Dataset({
        features: ['id', 'title', 'context', 'question', 'answers', 'metadata'],
        num_rows: 3000
    })
})
```

See issues: 
- https://huggingface.co/datasets/adversarial_qa/discussions/3
- https://huggingface.co/datasets/beans/discussions/4

This is a regression because it was previously fixed. See:
- #6196
- #5728