default Image/AudioFolder infers labels when there is no metadata files even if there is only one dir
{'login': 'polinaeterna', 'id': 16348744, 'node_id': 'MDQ6VXNlcjE2MzQ4NzQ0', 'avatar_url': 'https://avatars.githubusercontent.com/u/16348744?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/polinaeterna', 'html_url': 'https://github.com/polinaeterna', 'followers_url': 'https://api.github.com/users/polinaeterna/followers', 'following_url': 'https://api.github.com/users/polinaeterna/following{/other_user}', 'gists_url': 'https://api.github.com/users/polinaeterna/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/polinaeterna/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/polinaeterna/subscriptions', 'organizations_url': 'https://api.github.com/users/polinaeterna/orgs', 'repos_url': 'https://api.github.com/users/polinaeterna/repos', 'events_url': 'https://api.github.com/users/polinaeterna/events{/privacy}', 'received_events_url': 'https://api.github.com/users/polinaeterna/received_events', 'type': 'User', 'site_admin': False}
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{'login': 'polinaeterna', 'id': 16348744, 'node_id': 'MDQ6VXNlcjE2MzQ4NzQ0', 'avatar_url': 'https://avatars.githubusercontent.com/u/16348744?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/polinaeterna', 'html_url': 'https://github.com/polinaeterna', 'followers_url': 'https://api.github.com/users/polinaeterna/followers', 'following_url': 'https://api.github.com/users/polinaeterna/following{/other_user}', 'gists_url': 'https://api.github.com/users/polinaeterna/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/polinaeterna/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/polinaeterna/subscriptions', 'organizations_url': 'https://api.github.com/users/polinaeterna/orgs', 'repos_url': 'https://api.github.com/users/polinaeterna/repos', 'events_url': 'https://api.github.com/users/polinaeterna/events{/privacy}', 'received_events_url': 'https://api.github.com/users/polinaeterna/received_events', 'type': 'User', 'site_admin': False}
[{'login': 'polinaeterna', 'id': 16348744, 'node_id': 'MDQ6VXNlcjE2MzQ4NzQ0', 'avatar_url': 'https://avatars.githubusercontent.com/u/16348744?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/polinaeterna', 'html_url': 'https://github.com/polinaeterna', 'followers_url': 'https://api.github.com/users/polinaeterna/followers', 'following_url': 'https://api.github.com/users/polinaeterna/following{/other_user}', 'gists_url': 'https://api.github.com/users/polinaeterna/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/polinaeterna/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/polinaeterna/subscriptions', 'organizations_url': 'https://api.github.com/users/polinaeterna/orgs', 'repos_url': 'https://api.github.com/users/polinaeterna/repos', 'events_url': 'https://api.github.com/users/polinaeterna/events{/privacy}', 'received_events_url': 'https://api.github.com/users/polinaeterna/received_events', 'type': 'User', 'site_admin': False}]
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.

### 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`
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