Add the qml.Identity operation to PennyLane-Qiskit

[dime10]

Adds support for the qml.Identity operation to the Qiskit plugin, which was previously treated as a qml.QubitUnitary.

Support is added for both directions, that is Qiskit backends can handle PennyLane's Identity operation, as in this example:

import pennylane as qml
from pennylane_qiskit import BasicAerDevice

dev = BasicAerDevice(1)

@qml.qnode(dev)
def circuit():
    qml.Identity(wires=0)
    return qml.probs(wires=0)

circuit()

And the plugin converters correctly translate a Qiskit IGate to PennyLane's Idendity operation:

import pennylane as qml
from pennylane_qiskit import load
from qiskit import QuantumCircuit

recorder = qml.tape.OperationRecorder()

circ = QuantumCircuit(1)
circ.id(0)

with recorder:
    load(c)()

assert recorder.queue[0].name == "Identity"

Closes #153

[codecov]

Codecov Report

Merging #162 (7831013) into master (d6eff6e) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #162   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            7         7           
  Lines          313       313           
=========================================
  Hits           313       313           

Impacted Files	Coverage Δ
pennylane_qiskit/qiskit_device.py	100.00% <ø> (ø)

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[dime10]

[sc-9205]

[rmoyard]

Hi @dime10 thanks for the PR, I think you need to adapt load or add Identity operation to pennylane/ops/qubit/__init__.py, in order to make the converter work !

[dime10]

Hi @dime10 thanks for the PR, I think you need to adapt load or add Identity operation to pennylane/ops/qubit/init.py, in order to make the converter work !

Yes that change is coming but since it's a different repo it's not showing up here (yet) :)

[dime10]

PennyLaneAI/pennylane#1956 should make the converter test pass.

[rmoyard]

Hi @dime10, it looks good to me 💯 Just two comments, you could add Identity in the test of inverses and also update the changelog.

[rmoyard]

Thanks @dime10 !

[antalszava]

[sc-12053]