Image & Audio formatting for numpy/torch/tf/jax
Added support for image and audio formatting for numpy, torch, tf and jax.

For images, the dtype used is the one of the image (the one returned by PIL.Image), e.g. uint8

I also added support for string, binary and None types. In particular for torch and jax, strings are kept unchanged (previously it was returning an error because you can't create a tensor of strings)
_The documentation is not available anymore as the PR was closed or merged._ I just added a consolidation step so that numpy arrays or tensors of images are stacked together if the shapes match, instead of having lists of tensors

Feel free to review @mariosasko :) I added a few lines in the docs and reverted the ragged numpy array change :)

ready for another review @mariosasko !