Added missing label support for some QuantumCircuit gate-applying methods

[Bruno-Danti]

Summary

Problem:

Some common gate-applying QuantumCircuit methods, such as rz and s did not accept a label argument.

This led to some inconsistencies, such as qc.rx(np.pi, 0, label = "gate 0") working as intended but qc.rz(np.pi, 0, label = "gate 1") raising an error.

Fix:

Fix: Added a label to some of those methods and passed it to the self._append_standard_gate call

Test:

Added test/python/circuit/test_gate_labels.py to check that some of the fixed gates don't raise an error when invoked with a label, and that they retain the label afterwards

Details and comments

• There are a lot of gates in the QuantumCircuit class, and this fix is by no means extensive
• This pull request fixes issue Circuit Rz Gate has no "label" attribute #11723 and (partially) QuantumCircuit: some gates have no label attribute #14975
• Not sure if the added test is useful/necessary for the pull request/up to Qiskit's standards for tests

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[jakelishman]

Thanks for this, and sorry for the slow reply.

Please can you also add a release note (reno new <...>) , and you'll need to run the code formatter too (tox -e black).

[jakelishman]

Could we put this test function in an existing class in test/python/circuit/test_circuit_operations.py, just to avoid an extra stub file? There's no perfect place for it, but I think we can avoid the new file.

[jakelishman]

Qiskit uses the built-in unittest runner, so these should be self.assertEqual(qc.data[0].label, "rz_gate") to handle assertion failures / messages better.