Optimizer and Scheduler

[ndem0]

          I really like both `Optimizer` and `Scheduler` classes! I think the `hook` method can be made an abstract method for the base classes. In this way all the classes that inherit from those need to have this function (which is fundamental to attach the network parameters to optimize).

The only thing that I am concerned about is the back compatibility with Lightning, as by default Lightning will use built-in functions inherited from Torch Optimizers and Schedulers. Maybe in the base classes Optimizer and Scheduler we should inherit from torch base classes of optimizer and scheduler as well?

Originally posted by @dario-coscia in #312 (comment)

[dario-coscia]

@ndem0 I close the issue. In #401 new optimizer/schedulers fully back-compatible with lightning are introduced. Also there is the possibility to create new optimizers and scheduler!