Assert properties not specifics of DenseLayout results (#10901)

`DenseLayout` is a deterministic pass, so there's no risk of RNG
manipulations changing the output.  Internally, however, the use of the
sparse-matrix bandwidth-reduction algorithm in Scipy's
`reverse_cuthill_mckee` uses `numpy.argsort` internally with the
unstable default sorting algorithm, which means the output _can_ be
dependent on the way that sort is implemented.  The arrays that are
sorted are directly related to the input coupling map, and are likely to
include degeneracies, which pose problems when the implementation of the
unstable sort changes.  This was the case moving from Numpy 1.24 to
Numpy 1.25.

This commit instead changes the tests from asserting that a precise
layout was returned to asserting that the returned layout contains only
a connected subgraph of qubits.  The "most" connected component that
`DenseLayout` finds must be _at least_ connected, though this assertion
is not quite as strong as finding the _densest_.  The extra test
accounts for this weakening.

test/python/transpiler/test_dense_layout.py

@@ -12,6 +12,7 @@
 
 """Test the DenseLayout pass"""
 
+import itertools
 import unittest
 
 import numpy as np
@@ -43,6 +44,30 @@ def setUp(self):
         }
         self.target_19.add_instruction(CXGate(), instruction_props)
 
+    def test_finds_densest_component(self):
+        """Test that `DenseLayout` finds precisely the densest subcomponent of a coupling graph, not
+        just _any_ connected component."""
+        circuit = QuantumCircuit(5)
+        for a, b in itertools.permutations(circuit.qubits, 2):
+            circuit.cx(a, b)
+
+        # The map is a big long sparse line, except the middle 5 physical qubits are all completely
+        # connected, so `DenseLayout` should always choose those.
+        left_edge_qubits = range(0, 7)
+        middle_qubits = range(7, 12)
+        right_edge_qubits = range(12, 20)
+        cm = CouplingMap(
+            [(q, q + 1) for q in left_edge_qubits]
+            + [(q - 1, q) for q in right_edge_qubits]
+            + list(itertools.permutations(middle_qubits, 2))
+        )
+        cm.make_symmetric()
+        pass_ = DenseLayout(cm)
+        pass_(circuit)
+        layout = pass_.property_set["layout"]
+        used_qubits = {layout[q] for q in circuit.qubits}
+        self.assertEqual(used_qubits, set(middle_qubits))
+
     def test_5q_circuit_20q_coupling(self):
         """Test finds dense 5q corner in 20q coupling map."""
         qr = QuantumRegister(5, "q")
@@ -53,15 +78,14 @@ def test_5q_circuit_20q_coupling(self):
         circuit.cx(qr[0], qr[2])
 
         dag = circuit_to_dag(circuit)
-        pass_ = DenseLayout(CouplingMap(self.cmap20))
+        cm = CouplingMap(self.cmap20)
+        pass_ = DenseLayout(cm)
         pass_.run(dag)
 
         layout = pass_.property_set["layout"]
-        self.assertEqual(layout[qr[0]], 11)
-        self.assertEqual(layout[qr[1]], 10)
-        self.assertEqual(layout[qr[2]], 6)
-        self.assertEqual(layout[qr[3]], 5)
-        self.assertEqual(layout[qr[4]], 0)
+        actual = [layout[q] for q in circuit.qubits]
+        sub_map = cm.reduce(actual, check_if_connected=False)
+        self.assertTrue(sub_map.is_connected(), msg=f"chosen layout is not dense: {actual}")
 
     def test_6q_circuit_20q_coupling(self):
         """Test finds dense 5q corner in 20q coupling map."""
@@ -72,17 +96,14 @@ def test_6q_circuit_20q_coupling(self):
         circuit.cx(qr1[1], qr0[2])
 
         dag = circuit_to_dag(circuit)
-        pass_ = DenseLayout(CouplingMap(self.cmap20))
+        cm = CouplingMap(self.cmap20)
+        pass_ = DenseLayout(cm)
         pass_.run(dag)
 
         layout = pass_.property_set["layout"]
-
-        self.assertEqual(layout[qr0[0]], 11)
-        self.assertEqual(layout[qr0[1]], 10)
-        self.assertEqual(layout[qr0[2]], 6)
-        self.assertEqual(layout[qr1[0]], 5)
-        self.assertEqual(layout[qr1[1]], 1)
-        self.assertEqual(layout[qr1[2]], 0)
+        actual = [layout[q] for q in circuit.qubits]
+        sub_map = cm.reduce(actual, check_if_connected=False)
+        self.assertTrue(sub_map.is_connected(), msg=f"chosen layout is not dense: {actual}")
 
     def test_5q_circuit_19q_target_with_noise(self):
         """Test layout works finds a dense 5q subgraph in a 19q heavy hex target."""
@@ -96,11 +117,9 @@ def test_5q_circuit_19q_target_with_noise(self):
         pass_ = DenseLayout(target=self.target_19)
         pass_.run(dag)
         layout = pass_.property_set["layout"]
-        self.assertEqual(layout[qr[0]], 9)
-        self.assertEqual(layout[qr[1]], 3)
-        self.assertEqual(layout[qr[2]], 11)
-        self.assertEqual(layout[qr[3]], 15)
-        self.assertEqual(layout[qr[4]], 4)
+        actual = [layout[q] for q in circuit.qubits]
+        sub_map = self.target_19.build_coupling_map().reduce(actual, check_if_connected=False)
+        self.assertTrue(sub_map.is_connected(), msg=f"chosen layout is not dense: {actual}")
 
     def test_5q_circuit_19q_target_without_noise(self):
         """Test layout works finds a dense 5q subgraph in a 19q heavy hex target with no noise."""
@@ -117,11 +136,9 @@ def test_5q_circuit_19q_target_without_noise(self):
         pass_ = DenseLayout(target=noiseless_target)
         pass_.run(dag)
         layout = pass_.property_set["layout"]
-        self.assertEqual(layout[qr[0]], 1)
-        self.assertEqual(layout[qr[1]], 13)
-        self.assertEqual(layout[qr[2]], 0)
-        self.assertEqual(layout[qr[3]], 9)
-        self.assertEqual(layout[qr[4]], 3)
+        actual = [layout[q] for q in circuit.qubits]
+        sub_map = noiseless_target.build_coupling_map().reduce(actual, check_if_connected=False)
+        self.assertTrue(sub_map.is_connected(), msg=f"chosen layout is not dense: {actual}")
 
     def test_ideal_target_no_coupling(self):
         """Test pass fails as expected if a target without edge constraints exists."""
@@ -217,14 +234,13 @@ def test_5q_circuit_20q_with_if_else(self):
         circuit.cx(0, 4)
 
         dag = circuit_to_dag(circuit)
-        pass_ = DenseLayout(CouplingMap(self.cmap20))
+        cm = CouplingMap(self.cmap20)
+        pass_ = DenseLayout(cm)
         pass_.run(dag)
         layout = pass_.property_set["layout"]
-        self.assertEqual(layout[qr[0]], 11)
-        self.assertEqual(layout[qr[1]], 10)
-        self.assertEqual(layout[qr[2]], 6)
-        self.assertEqual(layout[qr[3]], 5)
-        self.assertEqual(layout[qr[4]], 0)
+        actual = [layout[q] for q in circuit.qubits]
+        sub_map = cm.reduce(actual, check_if_connected=False)
+        self.assertTrue(sub_map.is_connected(), msg=f"chosen layout is not dense: {actual}")
 
     def test_loose_bit_circuit(self):
         """Test dense layout works with loose bits outside a register."""
@@ -237,14 +253,13 @@ def test_loose_bit_circuit(self):
         circuit.cx(3, 0)
         circuit.cx(3, 1)
         dag = circuit_to_dag(circuit)
-        pass_ = DenseLayout(CouplingMap(self.cmap20))
+        cm = CouplingMap(self.cmap20)
+        pass_ = DenseLayout(cm)
         pass_.run(dag)
         layout = pass_.property_set["layout"]
-        self.assertEqual(layout[bits[0]], 11)
-        self.assertEqual(layout[bits[1]], 10)
-        self.assertEqual(layout[bits[2]], 6)
-        self.assertEqual(layout[bits[3]], 5)
-        self.assertEqual(layout[bits[4]], 0)
+        actual = [layout[q] for q in circuit.qubits]
+        sub_map = cm.reduce(actual, check_if_connected=False)
+        self.assertTrue(sub_map.is_connected(), msg=f"chosen layout is not dense: {actual}")
 
 
 if __name__ == "__main__":