title: Skip checksum computation in Imagefolder by default
comments: _The documentation is not available anymore as the PR was closed or merged._
body: Avoids having to set `ignore_verifications=True` in `load_dataset("imagefolder", ...)` to skip checksum verification and speed up loading.
  
  The user can still pass `DownloadConfig(record_checksums=True)` to not skip this part. 