Enhance Rot gates merging in merge_rotations transform

[andrijapau]

Feature details

We currently have,

from pennylane.transforms.optimization import merge_rotations

def qfunc():
    qml.Rot(-1, 0, 1, wires=0)
    qml.Rot(-1, 0, 1, wires=0)

>>> transformed_qfunc = merge_rotations(qfunc)
>>> qml.tape.make_qscript(transformed_qfunc)().operations
[Rot(-1, 0, 1, wires=[0])] # Should be [] since equivalent to I

But it should simplify this to an empty tape.

Implementation

Requires modifying how we check if the cumulative_angles are close to zero,

...
        # If we are tracing/jitting or differentiating, don't perform any conditional checks and
        # apply the operation regardless of the angles. Otherwise, only apply if
        # the rotation angle is non-trivial.
        if (
            qml.math.is_abstract(cumulative_angles)
            or qml.math.requires_grad(cumulative_angles)
            or not qml.math.allclose(cumulative_angles, 0.0, atol=atol, rtol=0)
        ):
            with QueuingManager.stop_recording():
                new_operations.append(
                    current_gate.__class__(*cumulative_angles, wires=current_gate.wires)
                )
...

As it doesn't catch the case where if the Rot cumulative angles are (X, 0, -X) when it is equivalent to the identity operator.

How important would you say this feature is?

1: Not important. Would be nice to have.

Additional information

It is important to note that single_qubit_fusion transform handles this case correctly.