Add optimizers to qiboml

[MatteoRobbiati]

Since in Qibo the optimizers refactor is very slow (due to many older applications and examples based on the old interface), I would suggest to move the optimizers here, so that we can start integrating the automatic differentiation into the optimizers.

The first two steps I am figuring out are:

1. integrating automatic diff with optimizers;
2. add a compile method to the models, which set optimizer and loss_function, and then write a model.fit method on top of the optimizers structure.

[alecandido]

I know this is not yet a Pytest suite, but it could be a starting point. I'd suggest you to immediately place it in tests/ top-level.