Base calibrations

[eggerdj]

Summary

This PR introduces the BaseCalibrations abstract base class.
This class introduces the minimal needed interface for calibration experiments to work.

Details and comments

This change is necessary as it will allow us to pursue the development of calibrations and keep ourselves aligned with developments in Qiskit. For example, the Target is the preferred way of creating backends. Currently, the Calibrations is based on the InstructionScheduleMap and it may be better to introduce a calibrations class that works with Target instead. This PR will allow us to, for example, and if desired

• introduce a calibrations V2 class
• gracefully deprecate calibrations in favor a class that is more aligned with recent developments in Qiskit

[nkanazawa1989]

Thanks Daniel. Overall this looks good to me, but the standpoint of this base class is bit unclear. I think this abstract class must define attributes and methods which are used by calibration experiments, so that we can safely implement new calibration agent without breaking experiment library. In this sense, this could be also a Protocol.

[itoko]

Thanks @eggerdj. It makes sense to me to have an interface class (BaseCalibrations) for calibration experiment classes for the smooth deprecation of current Calibrations class in the future. That said, the interface will last long and possibly affect the implementation of unseen another Calibrations (other than V2). I think we have enough time to explore a better interface and don't need to rush to merge it. So I'd like to see a concrete example of calibrations V2 class first. It is difficult for me to check if an abstract class is well-designed without having at least two subclasses. Also I think BaseCalibrations don't need to be just a subset of the existing Calibrations. It may be a good timing to consider minor improvement of Calibrations through following the new interface (I think Naoki's suggestions indicate that).

[eggerdj]

Marking as "On Hold" until we get more certainty on the required interface.

[yaelbh]

Note that this sentence is not finished. Also, what do you mean with "Parameters may be linked"? There's only one parameter here,

[nkanazawa1989]

This indicates parameter update can affect other schedules since it's linked.