Better support for the Identity gate

[andreamari]

Currently a Qiskit IGate is converted to a qml.ops.ops.QubitUnitary.
The reason is that qml.ops.Identity is only supported in PennyLane as an observable. Likely, in the future it will be supported as a an operation too. There is already an issue for this PennyLaneAI/pennylane#1632 .

So, as soon as PennyLaneAI/pennylane#1632 will be fixed, it would be nice to properly support the Identity gate also in this plug-in. It should be enough to add a new line in this conversion map:

pennylane-qiskit/pennylane_qiskit/qiskit_device.py

Line 36 in 043c544

QISKIT_OPERATION_MAP = {

[CatalinaAlbornoz]

Hi @andreamari, thank you very much for bringing this up! I agree that it would be useful to support it in the plugin. We will note it down!

[andreamari]

Thanks @dime10 !