`slide_dataset()` forgets to create `.../paddings`

Change

```python
makedirs(f"{output_folder}/images", exist_ok=True)
makedirs(f"{output_folder}/labels", exist_ok=True)
```

to

```python
makedirs(f"{output_folder}/images", exist_ok=True)
if supervised:
    makedirs(f"{output_folder}/labels", exist_ok=True)
makedirs(f"{output_folder}/paddings", exist_ok=True)
```