Make unrealized SingleQubitQPDGate has definition of None rather …

…than error

Fixes #417 according to #417 (comment)

circuit_knitting/cutting/qpd/instructions/qpd_gate.py

@@ -14,14 +14,12 @@
 
 from __future__ import annotations
 
-from abc import ABC, abstractmethod
-
 from qiskit.circuit import QuantumCircuit, Instruction, CircuitInstruction
 
 from ..qpd_basis import QPDBasis
 
 
-class BaseQPDGate(Instruction, ABC):
+class BaseQPDGate(Instruction):
     """Base class for a gate to be decomposed using quasiprobability decomposition."""
 
     def __init__(
@@ -101,11 +99,6 @@ def __eq__(self, other):
             and self.label == other.label
         )
 
-    @abstractmethod
-    def _define(self) -> None:
-        """Generate a decomposed gate."""
-        raise NotImplementedError  # pragma: no cover
-
 
 class TwoQubitQPDGate(BaseQPDGate):
     """Two qubit gate to be decomposed using quasiprobability decomposition."""
@@ -198,9 +191,10 @@ def _set_qubit_id(self, qubit_id: int) -> None:
 
     def _define(self) -> None:
         if self.basis_id is None:
-            raise ValueError(
-                "Missing 'basis_id': unable to realize SingleQubitQPDGate."
-            )
+            # With basis_id is not set, it does not make sense to define this
+            # operation in terms of more fundamental instructions, so we have
+            # self.definition remain as None.
+            return
         qc = QuantumCircuit(1)
         base = self.basis.maps[self.basis_id]
         for op in base[self.qubit_id]:

test/cutting/qpd/instructions/test_qpd_gate.py

@@ -92,12 +92,7 @@ def test_qubit_id_out_of_range(self):
     def test_missing_basis_id(self):
         maps = [([XGate()], [YGate()])]
         basis = QPDBasis(maps, [1.0])
-        with pytest.raises(ValueError) as e_info:
-            SingleQubitQPDGate(basis=basis, qubit_id=0).definition
-        self.assertEqual(
-            "Missing 'basis_id': unable to realize SingleQubitQPDGate.",
-            e_info.value.args[0],
-        )
+        assert SingleQubitQPDGate(basis=basis, qubit_id=0).definition is None
 
     def test_compare_1q_and_2q(self):
         maps = [([XGate()], [YGate()])]