Avoid using rustworkx 0.16.0 methods in vf2_utils (backport Qiskit#14513) (Qiskit#14533)

* Avoid using rustworkx 0.16.0 methods in vf2_utils (Qiskit#14513)

* Avoid using rustworkx 0.16.0 methods in vf2_utils

In Qiskit#14218 the vf2_utils module was updated to use the
`PyDiGraph.neighbors_undirected()` method which was added in 0.16.0.
However that PR neglected to bump the minimum required version of
rustworkx to 0.16.0 from 0.15.0 which is the current minim version
listed. While we could bump the minimum version (see Qiskit#14507) to
rustworkx 0.16.0 using this method isn't strictly necessary and Qiskit#14218
was backported to stable branches and backporting a version bump is not
desireable. This commit instead just updates the rustworkx usage to use
APIs in 0.15.0.

This PR will need to be backported to stable/1.4 and stable/2.0 to fix
compatibility with the listed rustworkx requirement. However, in the
backport a fix note will be needed to document that the release fixes
compatibility with the listed requirement.

* Correctly handle duplicates across successors and predecessors

(cherry picked from commit 9aa1a29)

* Add release note

---------

Co-authored-by: Matthew Treinish <[email protected]>

Author: mergify[bot]

qiskit/transpiler/passes/layout/vf2_utils.py

@@ -203,7 +203,9 @@ def build_average_error_map(target, properties, coupling_map):
         coupling_map = target.build_coupling_map()
     if not built and coupling_map is not None:
         for qubit in range(num_qubits):
-            degree = len(set(coupling_map.graph.neighbors_undirected(qubit)))
+            neighbor_set = set(coupling_map.graph.successor_indices(qubit))
+            neighbor_set.update(coupling_map.graph.predecessor_indices(qubit))
+            degree = len(neighbor_set)
             avg_map.add_error((qubit, qubit), degree / num_qubits)
         for edge in coupling_map.graph.edge_list():
             avg_map.add_error(edge, (avg_map[edge[0], edge[0]] + avg_map[edge[1], edge[1]]) / 2)

releasenotes/notes/fix-rustworkx-compatibility-8736854c63091cc4.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed a compatibility issue with the minimum supported version of rustworkx,
+    0.15. With certain inputs the :class:`.VF2Layout` and
+    :class:`.VF2PostLayout` were previously using a rustworkx method that was
+    added in rustworkx 0.16.0 which would cause an error when using an older
+    rustworkx release which is listed as supported.