AerBackend.process_circuits fails if circuits contain different numbers of qubits

[CalMacCQ]

Fairly minimal example provided by Blake. I'm not immediately sure what the best fix is.

from pytket import Circuit
from pytket.extensions.qiskit import AerBackend

tket_circuits = [
    Circuit(2, 2).H(0).CX(0, 1).Measure(0, 0).Measure(1, 1),
    Circuit(2, 2).H(1).CX(0, 1).Measure(0, 0).Measure(1, 1),
    Circuit(3, 3).H(0).CX(0, 1).CX(1, 2).Measure(0, 0).Measure(1, 1).Measure(2, 2),
]

aer_b = AerBackend()

print("Testing AerBackend same number of qubits")
handles = aer_b.process_circuits(tket_circuits[:2], n_shots=100)
counts = [aer_b.get_result(h).get_counts() for h in handles]

print("Testing AerBackend different number of qubits")

handles = aer_b.process_circuits(tket_circuits, n_shots=100)
counts = [aer_b.get_result(h).get_counts() for h in handles]

Output

Testing AerBackend same number of qubits
Testing AerBackend different number of qubits
Traceback (most recent call last):
  File "/Users/callum/work_projects/test-dir/.venv/lib/python3.11/site-packages/pytket/extensions/qiskit/backends/aer.py", line 328, in get_result
    return super().get_result(handle)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/callum/work_projects/test-dir/.venv/lib/python3.11/site-packages/pytket/backends/backend.py", line 347, in get_result
    raise CircuitNotRunError(handle)
pytket.backends.backend_exceptions.CircuitNotRunError: Circuit corresponding to ResultHandle('81269adf-afa7-4ee0-b4c2-bdea5844634a', 0, 2, 'null') has not been run by this backend instance.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/callum/work_projects/test-dir/support_issue.py", line 19, in <module>
    counts = [aer_b.get_result(h).get_counts() for h in handles]
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/callum/work_projects/test-dir/support_issue.py", line 19, in <listcomp>
    counts = [aer_b.get_result(h).get_counts() for h in handles]
              ^^^^^^^^^^^^^^^^^^^
  File "/Users/callum/work_projects/test-dir/.venv/lib/python3.11/site-packages/pytket/extensions/qiskit/backends/aer.py", line 346, in get_result
    self._cache[ResultHandle(jobid, circ_index, qubit_n, ppc)][
    ~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
KeyError: ResultHandle('81269adf-afa7-4ee0-b4c2-bdea5844634a', 2, 2, 'null')

[Btrainwilson]

I believe the issue starts in line 342 of pytket/extensions/qiskit/aer.py.

It tries super().get_result -> pytket.backend.Backend.get_result()
Which throws because "result" is not a key in self._cache[handle] for the AerBackend, as it is never assigned in line 325
Then, during the error handling in 343 onward, qubit_n is assigned (344) based on the first handle and is assumed across all handles in the loop in 352.