@qml.qnode(qubit_device_2_wires, interface='tf')
def circuit(w, x=None, y=None):
    qml.RX(x, wires=[0])
    qml.RX(y, wires=[1])
    return (qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliZ(1)))
@qml.qnode(qubit_device_2_wires, interface='tf')
def circuit(w, x=None, y=None):
    qml.RX(x, wires=[0])
    qml.RX(y, wires=[1])
    return (qml.expval(qml.PauliZ(0)), qml.expval(qml.PauliZ(1)))