Promote Instruction metadata to class attributes.

[kdk]

Summary

Currently, some useful details about each instruction or gate class are only available inside each instruction's __init__ method. e.g.

class HGate():
    def __init__(self):
        super().__init__('h', 1, [], num_ctrl_qubits=1)

meaning that when given an Instruction subclass, the only way to find out which instruction it is is to instantiate it and then query its name, num_qubits, etc. This PR promotes name, num_qubits, num_clbits, and num_parameters to be class attributes when known in advance (defaulting to None when these cannot be known until the gate is created.)

Details and comments

[ajavadia]

generally looks good. but shouldn't these be private and accessible via getters? like someone can instantiate a Hadamard now, then change num_qubits to 2, and it does not raise an error i think.

[ajavadia]

should there be num_params=0?

[ajavadia]

num_params=0? Otherwise it defaults to None, which seems like by definition means "unknown number of parameters".

[ajavadia]

num_params=1? Essentially this should identical to diagonal gate - both take a list of numbers.

[kdk]

When this is resolved, we can also remove the code to pick example gates from #4442 (comment) .

[mtreinish]

@kdk is there any update on this?

[kdk]

@kdk is there any update on this?

I think this is mostly subsumed by #6883 . Once that merges, I'll revisit to see if there's anything left to be done here.

[mtreinish]

I'm going to close this since it's been idle for 2.5 years and the plans around the instruction model has changed with Qiskit/RFCs#25 I think we'll need to start from scratch to do this assuming it works with the plans around operations