Fix instruction durations in transpile() with BackendV2 (#8001)

* Fix instruction durations in transpile() with BackendV2

When running transpile() with BackendV2 based backends the instruction
durations property from the backend would not be processed correctly
resulting in the absence of the default durations for instructions
supported on the target backend. This commit fixes this by correctly
handling BackendV2 based backends and using those instruction durations
by default for transpile().

* Update test/python/compiler/test_transpiler.py

Co-authored-by: Kevin Hartman <kevin@hart.mn>

Co-authored-by: Kevin Hartman <kevin@hart.mn>
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

qiskit/compiler/transpiler.py

@@ -921,11 +921,15 @@ def _parse_instruction_durations(backend, inst_durations, dt, circuits):
     take precedence over backend durations, but be superceded by ``inst_duration``s.
     """
     if not inst_durations:
-        backend_durations = InstructionDurations()
-        try:
-            backend_durations = InstructionDurations.from_backend(backend)
-        except AttributeError:
-            pass
+        backend_version = getattr(backend, "version", 0)
+        if backend_version <= 1:
+            backend_durations = InstructionDurations()
+            try:
+                backend_durations = InstructionDurations.from_backend(backend)
+            except AttributeError:
+                pass
+        else:
+            backend_durations = backend.instruction_durations
 
     durations = []
     for circ in circuits:

releasenotes/notes/fix-transpile-backendv2-durations-dbc85688564cc271.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed an issue with the :func:`~.transpile` function when run with a
+    :class:`~.BackendV2` based backend and setting the ``scheduling_method``
+    keyword argument. Previously, the function would not correctly process
+    the default durations of the instructions supported by the backend which
+    would lead to an error.

test/python/compiler/test_transpiler.py

@@ -35,7 +35,7 @@ from qiskit.converters import circuit_to_dag
 from qiskit.circuit.library import CXGate, U3Gate, U2Gate, U1Gate, RXGate, RYGate, RZGate, UGate
 from qiskit.circuit.measure import Measure
 from qiskit.test import QiskitTestCase
-from qiskit.test.mock import FakeMelbourne, FakeRueschlikon, FakeAlmaden
+from qiskit.test.mock import FakeMelbourne, FakeRueschlikon, FakeAlmaden, FakeMumbaiV2
 from qiskit.transpiler import Layout, CouplingMap
 from qiskit.transpiler import PassManager
 from qiskit.transpiler.target import Target
@@ -1242,6 +1242,18 @@ class TestTranspile(QiskitTestCase):
         out = transpile(qc, dt=1e-9)
         self.assertEqual(out.data[0][0].unit, "dt")
 
+    def test_scheduling_backend_v2(self):
+        """Test that scheduling method works with Backendv2."""
+        qc = QuantumCircuit(2)
+        qc.h(0)
+        qc.cx(0, 1)
+        qc.measure_all()
+
+        backend = FakeMumbaiV2()
+        out = transpile([qc, qc], backend, scheduling_method="alap")
+        self.assertIn("delay", out[0].count_ops())
+        self.assertIn("delay", out[1].count_ops())
+
     @data(1, 2, 3)
     def test_no_infinite_loop(self, optimization_level):
         """Verify circuit cost always descends and optimization does not flip flop indefinitely."""