[BUG] Using qml.apply on measurementProcess separated by commas returns ()

[austingmhuang]

Expected behavior

I am trying to use qml.apply to write test cases, specifically to parametrize across many different measurementProcesses. Some of these MPs look something like this:

dev = qml.device("default.qubit", wires=2, shots=1024)

op = qml.sample(qml.X(0) + qml.Y(0)), qml.sample(qml.X(0))
@qml.qnode(dev)
def circuit():
    qml.RX(np.pi / 3, 0)
    qml.RZ(np.pi / 3, 0)
    return qml.apply(op)

circuit()

This circuit should raise an error.

Actual behavior

It returns () without any indication of anything going wrong.

Additional information

No response

Source code

No response

Tracebacks

No response

System information

Name: PennyLane
Version: 0.37.0.dev0
Summary: PennyLane is a cross-platform Python library for quantum computing, quantum machine learning, and quantum chemistry. Train a quantum computer the same way as a neural network.
Home-page: https://github.com/PennyLaneAI/pennylane
Author: 
Author-email: 
License: Apache License 2.0
Location: /Users/austin.huang/pennylane/pennylane-qiskit/venv/lib/python3.11/site-packages
Requires: appdirs, autograd, autoray, cachetools, networkx, numpy, packaging, pennylane-lightning, requests, rustworkx, scipy, semantic-version, toml, typing-extensions
Required-by: PennyLane_Lightning

Platform info:           macOS-14.5-arm64-arm-64bit
Python version:          3.11.9
Numpy version:           1.26.4
Scipy version:           1.13.1
Installed devices:
- qiskit.aer (PennyLane-qiskit-0.35.0.dev0)
- qiskit.basicaer (PennyLane-qiskit-0.35.0.dev0)
- qiskit.ibmq (PennyLane-qiskit-0.35.0.dev0)
- qiskit.ibmq.circuit_runner (PennyLane-qiskit-0.35.0.dev0)
- qiskit.ibmq.sampler (PennyLane-qiskit-0.35.0.dev0)
- qiskit.remote (PennyLane-qiskit-0.35.0.dev0)
- default.clifford (PennyLane-0.37.0.dev0)
- default.gaussian (PennyLane-0.37.0.dev0)
- default.mixed (PennyLane-0.37.0.dev0)
- default.qubit (PennyLane-0.37.0.dev0)
- default.qubit.autograd (PennyLane-0.37.0.dev0)
- default.qubit.jax (PennyLane-0.37.0.dev0)
- default.qubit.legacy (PennyLane-0.37.0.dev0)
- default.qubit.tf (PennyLane-0.37.0.dev0)
- default.qubit.torch (PennyLane-0.37.0.dev0)
- default.qutrit (PennyLane-0.37.0.dev0)
- default.qutrit.mixed (PennyLane-0.37.0.dev0)
- default.tensor (PennyLane-0.37.0.dev0)
- null.qubit (PennyLane-0.37.0.dev0)
- lightning.qubit (PennyLane_Lightning-0.36.0)

Existing GitHub issues

• [X] I have searched existing GitHub issues to make sure the issue does not already exist.

[albi3ro]

Adding an error to qml.queuing.process_queue.