Preprocessing variables not defined as buffers of model

[EnricoTrizio]

When preprocessing is included in the model, preprocessing-related variables, which should be buffers for the model, are not passed as they should. This raises problems when we try to do model.to(dtype/device).

We could change the way preprocessing is passed to the model. Instead of as an attribute, we can use a property and define a set_preprocessing method which automatically loads the buffers of the preprocessing module into the model one (and remove them if preprocessing is set to None again).

We should also modify the Transform classes in the transform branch accordingly to have the needed variables saved as buffers, of course.

What do you think @andrrizzi?

Maybe we can also use the new approach to set the in/out_features and the example_input in the CV model in case of pre/postprocessing, maybe