[WIP][Bug Fix] Fixing the pow method on qml.TShift

[willjmax]

This PR fixes issue #6355 by providing proper pow implementations for qml.TShift, qml.TAdd, and qml.TClock. Each of these operators implement pow in the following way.

def pow(self, z):
    return super().pow(z % 3)

This is calling the pow method on Operator which handles the cases for z==0 and z==1 but returns a PowUndefinedError for any other value of z. This leads to an error for TShift, TAdd, and TClock when raising to the second power. This PR implements the case when z is a non-negative integer by returning a list containing z copies of the operator.

[github-actions]

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