Enforce passing by keyword argument in QNode and qml.execute (#6610)

**Context:**

Passing too many arguments positionally is error prone. It can be
difficult to remember what the proper order of the arguements is, and it
can be easy to reorder keyword arguments.

**Description of the Change:**

Start mandating that various keyword arguments *are* passed as keyword
arguments in `QNode` and `qml.execute`

**Benefits:**

We forbid entire classes of mistakes.

**Possible Drawbacks:**

**Related GitHub Issues:**

doc/releases/changelog-dev.md

@@ -90,6 +90,9 @@
   visualizations, allowing global and per-wire customization with options like `color`, `linestyle`, and `linewidth`.
   [(#6486)](https://github.com/PennyLaneAI/pennylane/pull/6486)
 
+* `QNode` and `qml.execute` now forbid certain keyword arguments from being passed positionally.
+  [(#6610)](https://github.com/PennyLaneAI/pennylane/pull/6610)
+
 * Shortened the string representation for the `qml.S`, `qml.T`, and `qml.SX` operators.
   [(#6542)](https://github.com/PennyLaneAI/pennylane/pull/6542)
 

pennylane/workflow/execution.py

@@ -249,6 +249,7 @@ def execute(
     device: Union["qml.devices.LegacyDevice", "qml.devices.Device"],
     diff_method: Optional[Union[Callable, str, qml.transforms.core.TransformDispatcher]] = None,
     interface: Optional[str] = "auto",
+    *,
     transform_program=None,
     inner_transform=None,
     config=None,
@@ -411,9 +412,16 @@ def cost_fn(params, x):
 
     gradient_kwargs = gradient_kwargs or {}
     mcm_config = mcm_config or {}
-    config = config or _get_execution_config(
-        diff_method, grad_on_execution, interface, device, device_vjp, mcm_config, gradient_kwargs
-    )
+    if not config:
+        config = qml.devices.ExecutionConfig(
+            interface=interface,
+            gradient_method=diff_method,
+            grad_on_execution=None if grad_on_execution == "best" else grad_on_execution,
+            use_device_jacobian_product=device_vjp,
+            mcm_config=mcm_config,
+            gradient_keyword_arguments=gradient_kwargs,
+        )
+        config = device.setup_execution_config(config)
 
     is_gradient_transform = isinstance(diff_method, qml.transforms.core.TransformDispatcher)
     transform_program, inner_transform = _make_transform_programs(
@@ -601,18 +609,3 @@ def _make_transform_programs(
             transform_program = device.preprocess_transforms(config)
 
     return transform_program, inner_transform
-
-
-def _get_execution_config(
-    diff_method, grad_on_execution, interface, device, device_vjp, mcm_config, gradient_kwargs
-):
-    """Helper function to get the execution config."""
-    config = qml.devices.ExecutionConfig(
-        interface=interface,
-        gradient_method=diff_method,
-        grad_on_execution=None if grad_on_execution == "best" else grad_on_execution,
-        use_device_jacobian_product=device_vjp,
-        mcm_config=mcm_config,
-        gradient_keyword_arguments=gradient_kwargs,
-    )
-    return device.setup_execution_config(config)

pennylane/workflow/qnode.py

@@ -607,6 +607,7 @@ def __init__(
         device: SupportedDeviceAPIs,
         interface: SupportedInterfaceUserInput = "auto",
         diff_method: Union[TransformDispatcher, SupportedDiffMethods] = "best",
+        *,
         grad_on_execution: Literal[True, False, "best"] = "best",
         cache: Union[Cache, Literal["auto", True, False]] = "auto",
         cachesize: int = 10000,