[BUG] `default.qubit.autograd` returns a tensor with `requires_grad=True`

Expected behavior

The output of circuit execution is a tensor that has the requires_grad attribute set to True or False for all devices.

Most devices output a tensor with requires_grad=False:

import pennylane as qml
from pennylane.devices import DefaultQubit

dev = qml.device('lightning.qubit', wires=1)

@qml.qnode(dev)
def circuit():
    qml.PauliX(0)
    return qml.expval(qml.PauliX(0))
    
circuit()

tensor(0., requires_grad=False)

Actual behavior

The default.qubit.autograd device outputs a tensor with requires_grad=False:

import pennylane as qml
from pennylane.devices import DefaultQubit

dev = qml.device('default.qubit.autograd', wires=1)

@qml.qnode(dev)
def circuit():
    qml.PauliX(0)
    return qml.expval(qml.PauliX(0))
    
circuit()

tensor(0., requires_grad=True)

With a tape:

import pennylane as qml
from pennylane.devices import DefaultQubit

device = qml.device('default.qubit.autograd', wires=1)

with qml.tape.QuantumTape() as tape:
    qml.PauliX(0)
    qml.expval(qml.PauliX(0))
    
res = device.execute(tape)

print(res, type(res), res.requires_grad)

[0.] <class 'pennylane.numpy.tensor.tensor'> True

Having requires_grad=True for the output seems to be unified for all devices when using the beta QNode:

import pennylane as qml
from pennylane.devices import DefaultQubit

dev = qml.device('lightning.qubit', wires=1)

@qml.beta.qnode(dev)
def circuit():
    qml.PauliX(0)
    return qml.expval(qml.PauliX(0))
    
circuit()

tensor(0., requires_grad=True)

Additional information

The issue to tackle would be to have a standardized output. Switching to the beta QNode seems to solve this, however, I’m not sure if having requires_grad=True for all outputs was intentional behaviour. If so, then this issue can be simply closed. 🙂

Source code

No response

Tracebacks

No response

System information

Name: PennyLane
Version: 0.19.0.dev0
Summary: PennyLane is a Python quantum machine learning library by Xanadu Inc.
Home-page: https://github.com/XanaduAI/pennylane
Author: None
Author-email: None
License: Apache License 2.0
Location: /pennylane
Requires: numpy, scipy, networkx, autograd, toml, appdirs, semantic-version, autoray, cachetools, pennylane-lightning
Required-by: PennyLane-Cirq, PennyLane-Orquestra, PennyLane-SF, pennylane-qulacs, PennyLane-IonQ, amazon-braket-pennylane-plugin, PennyLane-Forest, PennyLane-Honeywell, PennyLane-qiskit, PennyLane-AQT, PennyLane-Lightning, PennyLane-Qchem
Platform info:           Linux-5.11.0-37-generic-x86_64-with-glibc2.10
Python version:          3.8.5
Numpy version:           1.20.3
Scipy version:           1.7.1
Installed devices:
- cirq.mixedsimulator (PennyLane-Cirq-0.17.1)
- cirq.pasqal (PennyLane-Cirq-0.17.1)
- cirq.qsim (PennyLane-Cirq-0.17.1)
- cirq.qsimh (PennyLane-Cirq-0.17.1)
- cirq.simulator (PennyLane-Cirq-0.17.1)
- orquestra.forest (PennyLane-Orquestra-0.15.0)
- orquestra.ibmq (PennyLane-Orquestra-0.15.0)
- orquestra.qiskit (PennyLane-Orquestra-0.15.0)
- orquestra.qulacs (PennyLane-Orquestra-0.15.0)
- strawberryfields.fock (PennyLane-SF-0.16.0.dev0)
- strawberryfields.gaussian (PennyLane-SF-0.16.0.dev0)
- strawberryfields.gbs (PennyLane-SF-0.16.0.dev0)
- strawberryfields.remote (PennyLane-SF-0.16.0.dev0)
- strawberryfields.tf (PennyLane-SF-0.16.0.dev0)
- qulacs.simulator (pennylane-qulacs-0.17.0.dev0)
- ionq.qpu (PennyLane-IonQ-0.17.0.dev0)
- ionq.simulator (PennyLane-IonQ-0.17.0.dev0)
- braket.aws.qubit (amazon-braket-pennylane-plugin-1.4.1.dev0)
- braket.local.qubit (amazon-braket-pennylane-plugin-1.4.1.dev0)
- forest.numpy_wavefunction (PennyLane-Forest-0.17.0.dev0)
- forest.qvm (PennyLane-Forest-0.17.0.dev0)
- forest.wavefunction (PennyLane-Forest-0.17.0.dev0)
- honeywell.hqs (PennyLane-Honeywell-0.16.0.dev0)
- qiskit.aer (PennyLane-qiskit-0.18.0.dev0)
- qiskit.basicaer (PennyLane-qiskit-0.18.0.dev0)
- qiskit.ibmq (PennyLane-qiskit-0.18.0.dev0)
- aqt.noisy_sim (PennyLane-AQT-0.18.0)
- aqt.sim (PennyLane-AQT-0.18.0)
- lightning.qubit (PennyLane-Lightning-0.19.0.dev0)
- default.gaussian (PennyLane-0.19.0.dev0)
- default.mixed (PennyLane-0.19.0.dev0)
- default.qubit (PennyLane-0.19.0.dev0)
- default.qubit.autograd (PennyLane-0.19.0.dev0)
- default.qubit.jax (PennyLane-0.19.0.dev0)
- default.qubit.tf (PennyLane-0.19.0.dev0)
- default.qubit.torch (PennyLane-0.19.0.dev0)
- default.tensor (PennyLane-0.19.0.dev0)
- default.tensor.tf (PennyLane-0.19.0.dev0)

### 

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

[josh146]

Good to know 😅 yep if it is not blocking anything or causing any other bugs, not too high priority for now

[josh146]

One more question @antalszava, is this bug causing another bug? Just curious about priority in fixing this.