Fix ImageFolder with parameters drop_metadata=True and drop_labels=False (when metadata.jsonl is present)

Will fix #4621 

ImageFolder raises `KeyError: 'label'` with params `drop_metadata=True` and `drop_labels=False` (if there is at least one metadata.jsonl file a data directory). This happens because metadata files are collected inside `analyze()` function regardless of `drop_metadata` value. And then the following condition doesn't pass: https://github.com/huggingface/datasets/blob/master/src/datasets/packaged_modules/imagefolder/imagefolder.py#L167

So I suggest to double check it inside `analyze()` not to collect metadata files if they are not needed. (and labels too, to be consistent)

---
Also, I added a test to check if labels are inferred correctly from directories names in general (because we didn't have it) :)

_The documentation is not available anymore as the PR was closed or merged._

@lhoestq @mariosasko pls take a look at https://github.com/huggingface/datasets/pull/4622/commits/769e4c046a5bd5e3a4dbd09cfad1f4cf60677869. I modified `_generate_examples()` according to the same logic too: removed checking if `metadata_files` are not empty for the case when `self.config.drop_metadata=True` because I think we should be aligned with the config and preserve labels if `self.config.drop_labels=False` (the default value) and `self.config.drop_metadata=True` but `metadata_files` are passed. This is an extremely unlikely use case (when `self.config.drop_metadata=True`, but `metadata_files` are passed to `_generate_examples()`) since users usually do not use `_generate_examples()` alone but I believe it would be consistent to have the same behavior as in `_splits_generators()`. This change requires change in tests too if we suppose that we want to preserve labels (default value of `self.config.drop_labels` is False) when `self.config.drop_metadata=True`, even if `metadata_files` are for some reason provided (as it is done in tests). 

wdyt about this change?


@lhoestq it wouldn't raise an error if we check `example.keys() == {"image", "label"}` as test checks only `_generate_examples`, not `encode_example`. and in the implementation of this PR `_generate_examples` would return both `image` and `label` key in the case when `drop_metadata=True` and `drop_labels=False` (default) as it seems that we agreed on that :)

and on the other hand it would raise an error if `label` column is missing in _generate_examples when `drop_metadata=True` and `drop_labels=False`

by "it" i mean tests :D (`test_generate_examples_with_metadata_that_misses_one_image`, `test_generate_examples_with_metadata_in_wrong_location` and `test_generate_examples_drop_metadata`)

Perhaps we could make `self.config.drop_metadata = None` and `self.config.drop_labels = None` the defaults to see explicitly what the user wants. This would then turn into  `self.config.drop_metadata = False` and `self.config.drop_labels = True` if metadata files are present and `self.config.drop_metadata = True` and `self.config.drop_labels = False` if not. And if the user wants to have the `label` column alongside metadata columns, it can do so by passing `drop_labels = False` explicitely (in that scenario we have to check that the `label` column is not already present in metadata files). And maybe we can also improve the logging messages.

I find it problematic that the current implementation drops labels in some scenarios even if `self.config.drop_labels = False`, and the user doesn't have control over this behavior.

Let me know what you think.