Avoid iterating over nodes in Split2QUnitaries if there are no unitar…

…ies (#13384)

This commit adds a check to the start of the split_2q_unitaries rust
function that is the core of the Split2QUnitaries transpiler pass to do
a lookup if there are any `UnitaryGate` objects in the dag without
iterating over the dag. The pass only will split unitary gates so we
can avoid checking the name of each gate individually if we know there
aren't any up front.

crates/accelerate/src/split_2q_unitaries.rs

@@ -28,6 +28,9 @@ pub fn split_2q_unitaries(
     dag: &mut DAGCircuit,
     requested_fidelity: f64,
 ) -> PyResult<()> {
+    if !dag.get_op_counts().contains_key("unitary") {
+        return Ok(());
+    }
     let nodes: Vec<NodeIndex> = dag.op_nodes(false).collect();
 
     for node in nodes {