Switch scheduler callback

[GiovanniCanali]

Description

This PR fixes #734

Checklist

• Code follows the project’s Code Style Guidelines
• Tests have been added or updated
• Documentation has been updated if necessary
• Pull request is linked to an open issue

[dario-coscia]

I would keep the Scheduler callback, so that in the future, we can add other callbacks there

[GiovanniCanali]

This differs from #445, where we chose to name each file after the class it contains.

That said, your observation raises a valid point. My proposal is the following: within the callback directory, we create a dedicated scheduler subdirectory that, for now, will include only switch_scheduler.py, while leaving room for future additions. This prevents the file from becoming overly long as more scheduler-related callbacks are introduced.

The same structure would apply to optimizer_callback and processing_callback. In particular, the layout I have in mind is the following:

-- callback
-------- scheduler_callback
--------------- switch_scheduler.py
-------- optimizer_callback
--------------- switch_optimizer.py
--------- refinement
--------------- refinement_interface.py
--------------- r3_refinement.py
-------- processing_callback
--------------- metric_tracker.py
--------------- pina_progress_bar.py
--------------- normalizer_data_callback.py

Happy to hear your thoughts on this, @dario-coscia @FilippoOlivo @ndem0.

All these changes would be implemented in a dedicated PR.

[FilippoOlivo]

-- callback
-------- scheduler
--------------- switch_scheduler.py
-------- optimizer
--------------- switch_optimizer.py
--------- refinement
--------------- refinement_interface.py
--------------- r3_refinement.py
-------- processing
---------------  metric_tracker.py
--------------- pina_progress_bar.py
--------------- normalizer_data_callback.py

I prefer something like this (just remove callback on the subfolders)

[dario-coscia]

Very good! Maybe I would fuse scheduler and optimizer in optim directory (this is similar to from pina.optim import ...)