Only run a single inner routing trial for layout stage in SabreLayout (…

…#10741)

* Only run a single inner routing trial for layout stage in SabreLayout

In the SabreLayout routing pass we internally run sabre swap multiple
times forward and backwards over the circuit and transform the layout
based on the swaps that would be inserted by the routing pass.
Previously we were running multiple routing trials for this internal
execution of routing and then selecting the routing result with the
least numbers of swaps inserted. However, this heuristic of least swaps
does not necessarily result in a better layout. When increasing the
number of swap trials used for sabre layout the output quality could
result in more swaps being necessary. This points to the heuristic for
the best routing output to use for a layout is not the minimal number of
swaps. Until a more suitable heuristic can be developed for selecting
the best routing output to use for an intermediate layout transform this
commit removes the use of multiple trials for layout purposes and only
runs a single trial. We still use multiple swap trials for the final
routing as the swap count heuristic works there, but for layout we only
run a single trial.

* Avoid rayon use when running a single routing trial

crates/accelerate/src/sabre_layout.rs

@@ -156,8 +156,8 @@ fn layout_trial(
                 heuristic,
                 Some(seed),
                 &initial_layout,
-                num_swap_trials,
-                Some(run_swap_in_parallel),
+                1,
+                Some(false),
             );
             initial_layout = final_layout;
         }

test/python/transpiler/test_preset_passmanagers.py

@@ -917,68 +917,68 @@ def test_layout_tokyo_fully_connected_cx(self, level):
 
         sabre_layout = {
             0: ancilla[0],
-            1: qr[4],
-            2: ancilla[1],
-            3: ancilla[2],
-            4: ancilla[3],
-            5: qr[1],
-            6: qr[0],
-            7: ancilla[4],
-            8: ancilla[5],
-            9: ancilla[6],
-            10: qr[2],
-            11: qr[3],
-            12: ancilla[7],
-            13: ancilla[8],
-            14: ancilla[9],
-            15: ancilla[10],
-            16: ancilla[11],
+            1: ancilla[1],
+            2: ancilla[2],
+            3: ancilla[3],
+            4: ancilla[4],
+            5: qr[2],
+            6: qr[1],
+            7: ancilla[6],
+            8: ancilla[7],
+            9: ancilla[8],
+            10: qr[3],
+            11: qr[0],
+            12: ancilla[9],
+            13: ancilla[10],
+            14: ancilla[11],
+            15: ancilla[5],
+            16: qr[4],
             17: ancilla[12],
             18: ancilla[13],
             19: ancilla[14],
         }
 
         sabre_layout_lvl_2 = {
             0: ancilla[0],
-            1: qr[4],
-            2: ancilla[1],
-            3: ancilla[2],
-            4: ancilla[3],
-            5: qr[1],
-            6: qr[0],
-            7: ancilla[4],
-            8: ancilla[5],
-            9: ancilla[6],
-            10: qr[2],
-            11: qr[3],
-            12: ancilla[7],
-            13: ancilla[8],
-            14: ancilla[9],
-            15: ancilla[10],
-            16: ancilla[11],
+            1: ancilla[1],
+            2: ancilla[2],
+            3: ancilla[3],
+            4: ancilla[4],
+            5: qr[2],
+            6: qr[1],
+            7: ancilla[6],
+            8: ancilla[7],
+            9: ancilla[8],
+            10: qr[3],
+            11: qr[0],
+            12: ancilla[9],
+            13: ancilla[10],
+            14: ancilla[11],
+            15: ancilla[5],
+            16: qr[4],
             17: ancilla[12],
             18: ancilla[13],
             19: ancilla[14],
         }
 
         sabre_layout_lvl_3 = {
             0: ancilla[0],
-            1: qr[4],
-            2: ancilla[1],
-            3: ancilla[2],
-            4: ancilla[3],
-            5: qr[1],
-            6: qr[0],
-            7: ancilla[4],
-            8: ancilla[5],
-            9: ancilla[6],
-            10: qr[2],
-            11: qr[3],
-            12: ancilla[7],
-            13: ancilla[8],
-            14: ancilla[9],
-            15: ancilla[10],
-            16: ancilla[11],
+            1: ancilla[1],
+            2: ancilla[2],
+            3: ancilla[3],
+            4: ancilla[4],
+            5: qr[2],
+            6: qr[1],
+            7: ancilla[6],
+            8: ancilla[7],
+            9: ancilla[8],
+            10: qr[3],
+            11: qr[0],
+            12: ancilla[9],
+            13: ancilla[10],
+            14: ancilla[11],
+            15: ancilla[5],
+            16: qr[4],
             17: ancilla[12],
             18: ancilla[13],
             19: ancilla[14],

test/python/transpiler/test_sabre_layout.py

@@ -60,7 +60,7 @@ def test_5q_circuit_20q_coupling(self):
         pass_.run(dag)
 
         layout = pass_.property_set["layout"]
-        self.assertEqual([layout[q] for q in circuit.qubits], [18, 11, 13, 12, 14])
+        self.assertEqual([layout[q] for q in circuit.qubits], [11, 10, 16, 5, 17])
 
     def test_6q_circuit_20q_coupling(self):
         """Test finds layout for 6q circuit on 20q device."""
@@ -214,7 +214,7 @@ def test_layout_many_search_trials(self):
         self.assertIsInstance(res, QuantumCircuit)
         layout = res._layout.initial_layout
         self.assertEqual(
-            [layout[q] for q in qc.qubits], [7, 19, 14, 18, 10, 6, 12, 16, 13, 20, 15, 21, 1, 17]
+            [layout[q] for q in qc.qubits], [22, 7, 2, 12, 1, 5, 14, 4, 11, 0, 16, 15, 3, 10]
         )