set_format("np") no longer works for Image data
## Describe the bug
`dataset.set_format("np")` no longer works for image data, previously you could load the MNIST like this:

```python
dataset = load_dataset("mnist")
dataset.set_format("np")
X_train = dataset["train"]["image"][..., None] # <== No longer a numpy array
```
but now it doesn't work, `set_format("np")` seems to have no effect and the dataset just returns a list/array of PIL images instead of numpy arrays as requested.
