Add error if transform applied inside a qnode with program capture. (#2256)

**Context:**

We currently don't support applying transforms to a subcircuit. We need
to explicitly error out on this case.

For:
```
@qml.qjit
@qml.qnode(qml.device('lightning.qubit', wires=1))
@qml.transforms.cancel_inverses
def c():
    qml.X(0)
    qml.X(0)
    return qml.probs()

c(), print(c.mlir)
```
we would have gotten:
```
TypeError: No ir_type_handler for aval type: <class 'pennylane.measurements.capture_measurements._get_abstract_measurement.<locals>.AbstractMeasurement'>
```

Now we get:
```
NotImplementedError: transforms cannot currently be applied inside a QNode.
```
Which is much more helpful and informative.

**Description of the Change:**

`QFuncToPlxprInterpreter` now raises an error if it encounters a
transform.

**Benefits:**

Improved usability.

**Possible Drawbacks:**

**Related GitHub Issues:**

[sc-104320]

Author: albi3ro

doc/releases/changelog-dev.md

@@ -70,6 +70,9 @@
 
 <h3>Improvements 🛠</h3>
 
+* An error is now raised if a transform is applied inside a QNode when program capture is enabled.
+  [(#2256)](https://github.com/PennyLaneAI/catalyst/pull/2256)
+
 * A new ``"changed"`` option has been added to the ``keep_intermediate`` parameter of 
   :func:`~.qjit`. This option saves intermediate IR files after each pass,
   but only when the IR is actually modified by the pass.

frontend/catalyst/from_plxpr/qfunc_interpreter.py

@@ -32,6 +32,7 @@
 from pennylane.capture.primitives import ctrl_transform_prim as plxpr_ctrl_transform_prim
 from pennylane.capture.primitives import measure_prim as plxpr_measure_prim
 from pennylane.capture.primitives import pauli_measure_prim as plxpr_pauli_measure_prim
+from pennylane.capture.primitives import transform_prim
 from pennylane.ftqc.primitives import measure_in_basis_prim as plxpr_measure_in_basis_prim
 from pennylane.measurements import CountsMP
 
@@ -807,6 +808,11 @@ def calling_convention(*args_plus_qreg):
     return outvals
 
 
+@PLxPRToQuantumJaxprInterpreter.register_primitive(transform_prim)
+def _error_on_transform(*args, **kwargs):
+    raise NotImplementedError("transforms cannot currently be applied inside a QNode.")
+
+
 _special_op_bind_call = {
     qml.QubitUnitary: _qubit_unitary_bind_call,
     qml.GlobalPhase: _gphase_bind_call,

frontend/test/pytest/from_plxpr/test_from_plxpr.py

@@ -179,6 +179,21 @@ def c():
         with pytest.raises(NotImplementedError, match="not yet supported"):
             from_plxpr(jaxpr)()
 
+    def test_errors_transform_inside_qnode(self):
+        """Test that an error is raised if a transform is applied inside a transform."""
+
+        @qml.qnode(qml.device("lightning.qubit", wires=1))
+        @qml.transforms.cancel_inverses
+        def c():
+            return qml.expval(qml.Z(0))
+
+        jaxpr = jax.make_jaxpr(c)()
+
+        with pytest.raises(
+            NotImplementedError, match="transforms cannot currently be applied inside a QNode."
+        ):
+            from_plxpr(jaxpr)()
+
 
 class TestCatalystCompareJaxpr:
     """Test comparing catalyst and pennylane jaxpr for a variety of situations."""