Fix Split2QUnitaries dag manipulation

This commit fixes the dag handling to do the 1q unitary insertion.
Previously the dag manipulation was being done manually using the
insert_node_on_in_edges() rustworkx method. However as the original node
had 2 incoming edges for each qubit this caused the dag after running
the pass to become corrupted. Each of the new 1q unitary nodes would end
up with 2 incident edges and they would be in a sequence. This would result
in later passes not being able to correctly understand the state of the
circuit correctly. This was causing the unit tests to fail. This commit
fixes this by just using `substitute_node_with_dag()` to handle the
node substition, while doing it manually to avoid the overhead of
checking is probably possible, the case where a unitary is the product
of two 1q gates is not very common so optimizing it isn't super
critical.

Author: mtreinish

qiskit/transpiler/passes/optimization/split_2q_unitaries.py

@@ -48,24 +48,22 @@ def run(self, dag: DAGCircuit):
                 decomp._inner_decomposition.specialization
                 == TwoQubitWeylDecomposition._specializations.IdEquiv
             ):
+                new_dag = DAGCircuit()
+                new_dag.add_qubits(node.qargs)
+
                 ur = decomp.K1r
                 ur_node = DAGOpNode.from_instruction(
-                    CircuitInstruction(UnitaryGate(ur), qubits=(node.qargs[0],)), dag=dag
+                    CircuitInstruction(UnitaryGate(ur), qubits=(node.qargs[0],)), dag=new_dag
                 )
-                ur_node._node_id = dag._multi_graph.add_node(ur_node)
-                dag._increment_op("unitary")
-                dag._multi_graph.insert_node_on_in_edges(ur_node._node_id, node._node_id)
 
                 ul = decomp.K1l
                 ul_node = DAGOpNode.from_instruction(
-                    CircuitInstruction(UnitaryGate(ul), qubits=(node.qargs[1],)), dag=dag
+                    CircuitInstruction(UnitaryGate(ul), qubits=(node.qargs[1],)), dag=new_dag
                 )
-                ul_node._node_id = dag._multi_graph.add_node(ul_node)
-                dag._increment_op("unitary")
-                dag._multi_graph.insert_node_on_in_edges(ul_node._node_id, node._node_id)
-
-                dag.global_phase += decomp.global_phase
-                dag.remove_op_node(node)
+                new_dag._apply_op_node_back(ur_node)
+                new_dag._apply_op_node_back(ul_node)
+                new_dag.global_phase = decomp.global_phase
+                dag.substitute_node_with_dag(node, new_dag)
             elif (
                 decomp._inner_decomposition.specialization
                 == TwoQubitWeylDecomposition._specializations.SWAPEquiv

test/python/transpiler/test_split_2q_unitaries.py

@@ -163,7 +163,6 @@ def test_almost_identity(self):
         """Test that the pass handles QFT correctly."""
         qc = QuantumCircuit(2)
         qc.cp(pi * 2 ** -(26), 0, 1)
-        print(Operator(qc).data)
         pm = PassManager()
         pm.append(Collect2qBlocks())
         pm.append(ConsolidateBlocks())