default Image/AudioFolder infers labels when there is no metadata files even if there is only one dir
COLLABORATOR
Makes sense! For the last structure, we could count the path segments (delimited by "/" for URLs and `os.sep`  for local paths) to ensure all inferred labels are on the same level. Otherwise, I think it's safe to assume they are meaningless and ignore them.

2022-10-24T16:46:55Z
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### 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`