Parameters in MultiControlledX

As I was inspecting the MultiControlledX gate, I noticed how it printed out:

MultiControlledX(array([[0, 1],
       [1, 0]]), wires=[0, 1, 2, 3])

And was quite confused as to where the array came from. It turns out MultiControlledX inherits from ControlledQubitUnitary with the Unitary matrix as an X gate. This makes MultiControlledX a parametrized gate.

Worse yet, that gate is trainable:

>>> with qml.tape.JacobianTape() as tape:
...    qml.MultiControlledX(control_wires=(0,1,2), wires=(3))
>>> tape.trainable_params
{0}

This further indicates to me that how we treat never trainable parameters needs fixing in the long run.

In the short term, we should just remove the inheritance on ControlledQubitUnitary.

This would involve duplicating only relevant code in ControlledQubitUnitary and changing the inheritance to Operation.

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List of tasks for completion:

1. Change the MultiControlledX operation such that it doesn’t inherit from ControlledQubitUnitary anymore;
2. Include logic in MultiControlledX that is required for its full functionality;
3. Adjust any tests as required.

[tgag17]

Thank you @antalszava! I will try to tackle the issue and get back if I face any issue.

[albi3ro]

Thanks @tgag17 ! 👍

I assigned you the issue. Feel free to ask any additional questions here or when you open a PR.