default Image/AudioFolder infers labels when there is no metadata files even if there is only one dir
### Describe the bug

By default FolderBasedBuilder infers labels if there is not metadata files, even if it's meaningless (for example, they are in a single directory or in the root folder, see this repo as an example: https://huggingface.co/datasets/patrickvonplaten/audios

As this is a corner case for quick exploration of images or audios on the Hub. 

### Steps to reproduce the bug

If you have directory like this:

```
repo
   image1.jpg
   image2.jpg
   image3.jpg
```
or

```
repo
   data
      image1.jpg
      image2.jpg
      image3.jpg
```
doing `ds = load_dataset(repo)` would create `label` feature:
```python
print(ds["train"][0])
>> {'image': <PIL.JpegImagePlugin.JpegImageFile image mode=RGB size=500x375 at 0x7FB5326468E0>, 'label': 0}
```

Also, if you have the following structure:

```
repo
   data
      image1.jpg
      image2.jpg
      image3.jpg
   image4.jpg
   image5.jpg
   image6.jpg
```
it will infer two labels:

```python
print(ds["train"][0])
print(ds["train"][-1])
>> {'image': <PIL.JpegImagePlugin.JpegImageFile image mode=RGB size=500x375 at 0x7FB5326468E0>, 'label': 1}
>> {'image': <PIL.JpegImagePlugin.JpegImageFile image mode=RGB size=500x415 at 0x7FB5326555B0>, 'label': 0}
```

### Expected behavior

We should have only one base feature (Image/Audio) in such cases.

### Environment info

all versions of `datasets`