Remove list searches in CouplingMap.distance()

Previously, the CouplingMap.distance() method was checking whether the
qubit number was in the coupling graph by searching over a list for the
coupling map indices. This search becomes a bottleneck at large qubit
numbers because in the worst case it has to traverse the entire list to
find a match. This also is unecessary because the coupling map indices
are always an ordered range from 0 to n for n qubits. This commit fixes
this bottleneck by updating the check to just error if the index is out
of bounds for the size of the graph.

Related to Qiskit#5197

qiskit/transpiler/coupling.py

@@ -170,10 +170,11 @@ def distance(self, physical_qubit1, physical_qubit2):
         Raises:
             CouplingError: if the qubits do not exist in the CouplingMap
         """
-        if physical_qubit1 not in self.physical_qubits:
-            raise CouplingError("%s not in coupling graph" % (physical_qubit1,))
-        if physical_qubit2 not in self.physical_qubits:
-            raise CouplingError("%s not in coupling graph" % (physical_qubit2,))
+        size = self.size()
+        if physical_qubit1 >= size:
+            raise CouplingError("%s not in coupling graph" % physical_qubit1)
+        if physical_qubit2 >= size:
+            raise CouplingError("%s not in coupling graph" % physical_qubit2)
         if self._dist_matrix is None:
             self._compute_distance_matrix()
         return int(self._dist_matrix[physical_qubit1, physical_qubit2])