How Jax and Torch backends are used: we need tutorials

Created by: gcarleo

Once we finish up the interfaces (hopefully before the end of May), we should write a couple of jupyter notebook tutorials explaining how we can use Jax and Torch machines.

Ideally, we should also add a demonstration on the GPU (I guess with Jax, even though it's still a bit tricky to install on GPUs for the moment...).