Use numpy sum instead of comprehension in _score_heuristic (#5403)

* Use numpy sum instead of comprehension in _score_heuristic

In sabre_swap the comprehension in _score_heuristic() with the basic
heuristic starts to become a bottleneck as the size of the coupling map
increases. This probably becomes the top bottleneck in a transpilation
after #5316, #5183, #5294, #5267, and #5272 are merged. This commit is
an attempt to try and mitigate that a bit by using numpy native
operations instead of a python comprehension in sum(). If this is
insufficient we'll likely have to either avoid doing a sum like this or
drop down to a lower level with cython or rust and operate on the array
directly to remove this bottleneck.

* Make distance matrix a coupling map property

* Fix tests

* Fix lint

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/transpiler/coupling.py

@@ -140,6 +140,13 @@ def neighbors(self, physical_qubit):
         """
         return self.graph.neighbors(physical_qubit)
 
+    @property
+    def distance_matrix(self):
+        """Return the distance matrix for the coupling map."""
+        if self._dist_matrix is None:
+            self._compute_distance_matrix()
+        return self._dist_matrix
+
     def _compute_distance_matrix(self):
         """Compute the full distance matrix on pairs of nodes.
 

qiskit/transpiler/passes/routing/sabre_swap.py

@@ -314,8 +314,15 @@ def _score_heuristic(self, heuristic, front_layer, extended_set, layout, swap_qu
         the remaining virtual gates that must be applied.
         """
         if heuristic == 'basic':
-            return sum(self.coupling_map.distance(*[layout[q] for q in node.qargs])
-                       for node in front_layer)
+            if len(front_layer) > 1:
+                return self.coupling_map.distance_matrix[
+                    tuple(zip(*[[
+                        layout[q] for q in node.qargs] for node in front_layer]))].sum()
+            elif len(front_layer) == 1:
+                return self.coupling_map.distance(
+                    *[layout[q] for q in list(front_layer)[0].qargs])
+            else:
+                return 0
 
         elif heuristic == 'lookahead':
             first_cost = self._score_heuristic('basic', front_layer, [], layout)