Implementation of a pulse schedule block

[nkanazawa1989]

What is the expected behavior?

Pulse Schedule doesn't support parameterization of duration because it immediately evaluates the overlap of instruction over the channel timeslots. Here we implement new program representation called ScheduleBlock which consists of transform_policy: Enum and blocks: List[Union[Instruction, ScheduleBlock]]. The transform_policy represents one of transformation functions defined in qiskit.pulse.transforms and this transform is applied to underlying blocks when the schedule block is converted into Schedule. Because this new representation doesn't have explicit timeslot, it can involve instruction with parameterized duration, and the duration is assigned just before it is converted into schedule (i.e. lazy timeslot evaluation). The implementation of pulse builder is also updated to use this new representation. The ScheduleBlock has compatibility with the Schedule thus there is no usability impact by this update. In addition, Schedule will not be deprecated and coexist with ScheduleBlock in Qiskit Pulse.

This will be split into:

• Phase1: Remove timeslot dependency from Instruction
• Phase2: Add Call instruction to wrap an appended schedule component by pulse.call
• Phase3: Implement ScheduleBlock
• Phase4: Update pulse builder implementation