Shape error for tensor-like parameters in pauli arithmetic fixed (Addition to PR: #6562)

[ldi18]

Context:
I added Pauli string decompositions for non-parametrized gates in PR: #6562. In the initial version of PR: #6562 also included Pauli string decompositions for single-qubit parametrized gates - they had to be removed for now because of issues in CI testing - but the bug below remains relevant as it might surface again once more Pauli-reps are added:

Pennylane can exploit Pauli string representations when, for example, computing

(qml.RX(1.11, wires=0) @ qml.RY(1.23, wires=0)).matrix()

The example above works as intended, but for tensor-like parameters as in

(qml.RX([1.11, 2.11, 3.11], wires=0) @ qml.RY([1.23, 2.23, 3.23], wires=0)).matrix()     # shape error

or

(qml.RX(np.array([1.11, 2.11, 3.11]), wires=0) @ qml.RY(np.array([1.23, 2.23, 3.23]), wires=0)).matrix()     # shape error

a shape error is returned. The code above should work since RXand RYsupport tensor-like (flattened) parameters as it can be seen from

qml.RX([1.11, 2.11, 3.11], wires=0).matrix()    # works!

Description of the Change:

I added an outer product in pauli_arithmetic.

Benefits:

• (Future) Pauli representations of parametrized gates can be leveraged for the matrix conversion of products of ops.
• A Pauli representation of the PhaseShift gate could be added - this was not possible before as the PCPhase gate used the pauli_arithmetic, which gave an error in the pytests.

Possible Drawbacks:
None

Related GitHub Issues:
#6243 and PR: #6562