Fix incorrect gradient in qml.Select

[DSGuala]

Description of the Change:
Fixes issue with qml.Select that gave incorrect gradients when using parameter-shift:

>>> dev = qml.device('default.qubit', wires=4)
>>> @qml.qnode(dev, diff_method="parameter-shift")
>>> def circuit(x):
>>>     ops = [qml.RX(x, wires=2), qml.PauliX(wires=3), qml.PauliY(wires=2), qml.SWAP([2,3])]
>>>     qml.Select(ops, control_wires=(0,1))
>>>     return qml.expval(qml.PauliZ(2))
>>> qml.grad(circuit)(qml.numpy.array(0.5))
array(0.)

>>> dev = qml.device('default.qubit', wires=4)
>>> @qml.qnode(dev, diff_method="parameter-shift", expansion_strategy="device")
>>> def circuit(x):
>>>     ops = [qml.RX(x, wires=2), qml.PauliX(wires=3), qml.PauliY(wires=2), qml.SWAP([2,3])]
>>>     qml.Select(ops, control_wires=(0,1))
>>>     return qml.expval(qml.PauliZ(2))
>>> qml.grad(circuit)(qml.numpy.array(0.5))
-0.479425538604203

Benefits:
Solves differentiability issues.

Possible Drawbacks:
Adding more differentiable parameters to Select makes the circuit drawing wider:

0: ─╭Select(0.50,0.50,0.50)─┤     
1: ─├Select(0.50,0.50,0.50)─┤     
2: ─├Select(0.50,0.50,0.50)─┤  <Z>
3: ─╰Select(0.50,0.50,0.50)─┤  

[albi3ro]

Thanks for that change 👍 Looks good to me :)

[codecov]

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