pass along postselect mode

pennylane/devices/legacy_facade.py

@@ -391,11 +391,17 @@ def execute(self, circuits, execution_config=DefaultExecutionConfig):
             else self._device
         )
 
+        kwargs = {}
+        if dev.capabilities().get("supports_mid_measure", False):
+            kwargs["postselect_mode"] = execution_config.mcm_config.postselect_mode
+
         first_shot = circuits[0].shots
         if all(t.shots == first_shot for t in circuits):
-            results = _set_shots(dev, first_shot)(dev.batch_execute)(circuits)
+            results = _set_shots(dev, first_shot)(dev.batch_execute)(circuits, **kwargs)
         else:
-            results = tuple(_set_shots(dev, t.shots)(dev.batch_execute)((t,))[0] for t in circuits)
+            results = tuple(
+                _set_shots(dev, t.shots)(dev.batch_execute)((t,), **kwargs)[0] for t in circuits
+            )
 
         if dev is not self._device:
             self._update_original_device(dev)

tests/devices/test_legacy_facade.py

@@ -236,6 +236,28 @@ class MidMeasureDev(DummyDevice):
     assert qml.defer_measurements not in program
 
 
+@pytest.mark.parametrize("t_postselect_mode", ("hw-like", "fill-shots"))
+def test_pass_postselect_mode_to_dev(t_postselect_mode):
+    """test that postselect mode is passed to the target if it supports mid measure."""
+
+    class MidMeasureDev(DummyDevice):
+        """A dummy device that supports mid circuit measurements."""
+
+        _capabilities = {"supports_mid_measure": True}
+
+        def batch_execute(self, circuits, postselect_mode):
+            assert postselect_mode == t_postselect_mode
+            return tuple(0 for _ in circuits)
+
+    target = MidMeasureDev()
+    dev = LegacyDeviceFacade(target)
+
+    mcm_config = qml.devices.MCMConfig(postselect_mode=t_postselect_mode)
+    config = qml.devices.ExecutionConfig(mcm_config=mcm_config)
+
+    dev.execute(qml.tape.QuantumScript(), config)
+
+
 class TestGradientSupport:
     """Test integration with various kinds of device derivatives."""