Issue with inverse of QubitStateVector

[trbromley]

Describe the bug
An error is raised when using qml.QubitStateVector(...).inv() with any of the Qiskit devices.

The error is:

CircuitError: "Invalid param type <class 'complex'> for gate initialize_dg."

To Reproduce

import pennylane as qml
import numpy as np

dev = qml.device("qiskit.aer", wires=1)
state = np.array([1, 1]) / np.sqrt(2)

@qml.qnode(dev)
def f():
    qml.QubitStateVector(state, wires=0).inv()
    return qml.expval(qml.PauliZ(0))

f()

Expected behavior
The above error is not raised.

We could raise a different error because we don't want to support inverses of state preparations (see PennyLaneAI/pennylane#2666).

Additional context

The error itself is linked to the use of Qiskit's Initialize here, which does not support inverses.

This issue was found by @QFer when working on a separate plugin that inherits from the Qiskit plugin. The error occurs in this test in the device test suite when QubitStateVector is added as a supported operation.

[lillian542]

The inv() issue is no longer relevant, and this code runs with qml.adjoint(qml.QubitStateVector(state, wires=0)) instead. Not sure if that resolves the linked issue in PennyLane, but that seems like a separate question. @trbromley , can we close this?