refactor(datasets): Adjust datasets versions

[jafermarq]

HF's datasets 4.0 has been out for a couple of months, but it's not supported by flower-datasets officially yet.

• In HF datasets 4.x, the audio extra requires torch and torchcodec. Therefore, these have been added as optional extras.
• Minor tweaks to test due to changes in HF datasets 4.x
• CI tests now need to install torch which requires first freeing some space and installing ffmpeg (hence the changes in the workflows)