[Imagefolder] Docs + Don't infer labels from file names when there are metadata + Error messages when metadata and images aren't linked correctly
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_The documentation is not available anymore as the PR was closed or merged._ Merging this one since mario is off, I took care of adding some tests to make sure everything is fine. Will do the release after it
I updated the `docs/source/image_process.mdx` documentation and added an example for image captioning and object detection using `ImageFolder`.

While doing so I also improved a few aspects:
- we don't need to infer labels from file names when there are metadata - they can just be in the metadata if necessary
- raise informative error messages when metadata and images aren't linked correctly:
  - when an image is missing a metadata file
  - when a metadata file is missing an image

I added some tests for these changes as well

cc @mariosasko 
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