More Schedule-ScheduleBlock integration

[nkanazawa1989]

Summary

Currently the data representation of Schedule and ScheduleBlock are quite different and most of transform functions support only Schedule. Thus we always need to convert block to schedule before using these transforms. This conversion is irreversible and we lost alignment context due to this conversion.

This PR attempts to introduce ScheduleNode that has data, time, location as variable. With this we can redefine block (schedule) as a sequence of node with unassigned (assigned) node time. Note that we are still able to assign time to the nodes of the blocks (if all node durations are fixed), and we can schedule the block while keeping the alignment context. Thus we can use most of transformation function without converting it into schedule.

Note that we can update node time and node data (i.e. node is mutable object) without breaking the structure of schedule. Since each node is managed by unique location (uuid4) id.

This also unifies representation of schedule and block. Thus we can add schedule_to_block conversion with a special context of Frozen (this doesn't perform alignment but returns node sequence as-is since it's already scheduled).

Now the builder can return time-assigned nodes if possible.

Details and comments

This is an example of what can be done with this PR.

with pulse.build() as sched:
    pulse.shift_phase(1.57, pulse.DriveChannel(0))
    pulse.play(pulse.Gaussian(160, 0.1, 40), pulse.DriveChannel(0))
    with pulse.align_right():
        pulse.play(pulse.Gaussian(80, 0.1, 20), pulse.DriveChannel(1))
        pulse.play(pulse.Gaussian(160, 0.3, 40), pulse.DriveChannel(0))
    pulse.shift_phase(1.57, pulse.DriveChannel(1))
    with pulse.align_sequential():
        pulse.play(pulse.Gaussian(80, 0.1, 20), pulse.DriveChannel(2))
        pulse.shift_phase(1.57, pulse.DriveChannel(0))
        pulse.play(pulse.Gaussian(160, 0.1, 40), pulse.DriveChannel(1))
    pulse.shift_phase(1.57, pulse.DriveChannel(0))

filt_sched = filters.filter_instructions(sched, filters.with_intervals([[0, 300]]))

type(filt_sched)  # qiskit.pulse.schedule.ScheduleBlock

Since all sched instructions have fixed duration, we can assign node time when the builder returns the root block. We can filter returned sched by time interval without converting it into a schedule.

Comments:
Still this PR has lots of bugs and there is no unittest for transform of schedule block. Before going into detailed design and bug fixes, I'd like to hear feedback about this approach. If this doesn't make sense I'll just close this PR. It seems like we can completely integrate schedule and block at some point.

Since this is large PR it might be efficient to start from qiskit/pulse/schedule.py to grasp rough framework of this PR.

[nkanazawa1989]

Closed because this is not right direction.