Backward tests for all PINA models

[dario-coscia]

Is your feature request related to a problem? Please describe.
By now all the PINA models are tested only for the forward pass. I suggest to add a test also for the backward pass since it is usually used during training.

[guglielmopadula]

I have created a new branch and commited the tests. Let me know if they are ok.

[dario-coscia]

Very good! Please create a PR for approval