Add support for CSV metadata files to ImageFolder
_The documentation is not available anymore as the PR was closed or merged._ Cool thanks ! Maybe let's include this change after the refactoring from FolderBasedBuilder in #3963 to avoid dealing with too many unpleasant conflicts ? @lhoestq I resolved the conflicts (AudioFolder also supports CSV metadata now). Let me know what you think.
 @lhoestq Thanks for the suggestion! Indeed it makes more sense to use CSV as the default format in the folder-based builders.
Fix #4814