🎨 improved RL Predictor qcompile method signature (#176)

This PR improves the method signature of
`mqt.predictor.rl.helper.qcompile`.

src/mqt/predictor/rl/helper.py

@@ -68,6 +68,7 @@
     import importlib_metadata as metadata
     import importlib_resources as resources
 
+
 logger = logging.getLogger("mqt-predictor")
 
 
@@ -83,8 +84,8 @@
 
 def qcompile(
     qc: QuantumCircuit | str,
-    figure_of_merit: reward.figure_of_merit = "expected_fidelity",
-    device_name: str = "ibm_washington",
+    figure_of_merit: reward.figure_of_merit | None = "expected_fidelity",
+    device_name: str | None = "ibm_washington",
     predictor_singleton: rl.Predictor | None = None,
 ) -> tuple[QuantumCircuit, list[str]]:
     """Compiles a given quantum circuit to a device optimizing for the given figure of merit.
@@ -93,13 +94,19 @@ def qcompile(
         qc (QuantumCircuit | str): The quantum circuit to be compiled. If a string is given, it is assumed to be a path to a qasm file.
         figure_of_merit (reward.reward_functions, optional): The figure of merit to be used for compilation. Defaults to "expected_fidelity".
         device_name (str, optional): The name of the device to compile to. Defaults to "ibm_washington".
-        predictor_singleton (rl.Predictor, optional): A predictor object that is used for compilation. If None, a new predictor object is created. Defaults to None.
+        predictor_singleton (rl.Predictor, optional): A predictor object that is used for compilation to reduce compilation time when compiling multiple quantum circuits. If None, a new predictor object is created. Defaults to None.
 
     Returns:
         tuple[QuantumCircuit, list[str]] | bool: Returns a tuple containing the compiled quantum circuit and the compilation information. If compilation fails, False is returned.
     """
 
     if predictor_singleton is None:
+        if figure_of_merit is None:
+            msg = "figure_of_merit must not be None if predictor_singleton is None."
+            raise ValueError(msg)
+        if device_name is None:
+            msg = "device_name must not be None if predictor_singleton is None."
+            raise ValueError(msg)
         predictor = rl.Predictor(figure_of_merit=figure_of_merit, device_name=device_name)
     else:
         predictor = predictor_singleton

tests/compilation/test_predictor_rl.py

@@ -40,3 +40,11 @@ def test_qcompile_with_newly_trained_models(figure_of_merit: reward.figure_of_me
 
     assert isinstance(qc_compiled, QuantumCircuit)
     assert compilation_information is not None
+
+
+def test_qcompile_with_false_input() -> None:
+    qc = get_benchmark("dj", 1, 5)
+    with pytest.raises(ValueError, match="figure_of_merit must not be None if predictor_singleton is None."):
+        rl.helper.qcompile(qc, None, "ibm_washington")
+    with pytest.raises(ValueError, match="device_name must not be None if predictor_singleton is None."):
+        rl.helper.qcompile(qc, "expected_fidelity", None)