Drop labels in Image and Audio folders if files are on different levels in directory or if there is only one label
Will close https://github.com/huggingface/datasets/issues/5153

Drop labels by default (`drop_labels=None`) when:
* there are files on different levels of directory hierarchy by checking their path depth
* all files are in the same directory (=only one label was inferred)

First one fixes cases like this:
```
repo
   image3.jpg
   image4.jpg
   data
      image1.jpg
      image2.jpg
```
Second one fixes cases like this:
```
repo
   image1.jpg
   image2.jpg
   image3.jpg
```
This is mostly to fix the viewer for people who just drop images in the Hub interface into the root dir.

I added tests for both of the cases on local and remote files. **I also changed data files for old test on drop_labels** (`test_generate_examples_drop_labels`). The files I provide to `test_generate_examples_drop_labels` now has "canonical" classification structure (two dirs) in order not to change the logic of the test (=not to check these two cases addressed in the PR).

