Improve efficiency of CouplingMap.make_symmetric

Right now the CouplingMap.make_symmetric has a quadratic overhead. It
was iterating over a list of each edgess' endpoints in the graph
and for each edge it was iterating over the same list again to check
whether the reverse edge is present or not. The overhead for this for
large coupling maps can be quite large, for example with a 10497 qubit
heavy hex coupling map the time it took to run this method as part of
SabreLayout's initialization was 10x slower than actually running the
pass (a equally sized Bernstein-Vazirani circuit). Instead of doing an
O(n) contains check inside the loop this commit updates it to use a
set which will be an O(1) lookup. This should address the performance
issue with this method. In the future we should leverage the native
rustworkx method to do this, which is being added in:
Qiskit/rustworkx#814

qiskit/transpiler/coupling.py

@@ -234,9 +234,12 @@ def make_symmetric(self):
         """
         Convert uni-directional edges into bi-directional.
         """
+        # TODO: replace with PyDiGraph.make_symmetric() after rustworkx
+        # 0.13.0 is released.
         edges = self.get_edges()
+        edge_set = set(edges)
         for src, dest in edges:
-            if (dest, src) not in edges:
+            if (dest, src) not in edge_set:
                 self.add_edge(dest, src)
         self._dist_matrix = None  # invalidate
         self._is_symmetric = None  # invalidate