title: [Imagefolder] Docs + Don't infer labels from file names when there are metadata + Error messages when metadata and images aren't linked correctly
comments: _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
body: 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 