Sampler Batch pattern works via QiskitRuntimeService but errors on a fake backend in local mode

[garrison]

Describe the bug

The pattern to use Batch mode

with Batch(backend=backend):
    sampler = SamplerV2()

works properly via QiskitRuntimeService but fails with an error on a fake backend, which I discovered in CI at Qiskit/qiskit-addon-cutting#616.

Note that currently one can work around this by using an alternative pattern:

with Batch(backend=backend) as batch:
    sampler = SamplerV2(session=batch)

Steps to reproduce

from qiskit import QuantumCircuit
from qiskit.transpiler.preset_passmanagers import generate_preset_pass_manager
from qiskit_ibm_runtime import SamplerV2, Batch
from qiskit_ibm_runtime.fake_provider import FakeManilaV2

qc0 = QuantumCircuit(2, 2)
qc0.h(0)
qc0.cx(0, 1)
qc0.measure(range(2), range(2))

qc1 = QuantumCircuit(2, 2)
qc1.h(1)
qc1.cx(1, 0)
qc0.measure(range(2), range(2))

pass_manager = generate_preset_pass_manager(optimization_level=1, backend=backend)
qc0 = pass_manager.run(qc0)
qc1 = pass_manager.run(qc1)

backend = FakeManilaV2()
with Batch(backend=backend):
    sampler = SamplerV2()
    job0 = sampler.run([qc0])
    job1 = sampler.run([qc1])

print(job0.result()[0].data.c.array)
print(job1.result()[0].data.c.array)

raises the error

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[11], line 22
     20 backend = FakeManilaV2()
     21 with Batch(backend=backend):
---> 22     sampler = SamplerV2()
     23     job0 = sampler.run([qc0])
     24     job1 = sampler.run([qc1])

File ~/Qiskit/qiskit-ibm-runtime/qiskit_ibm_runtime/sampler.py:127, in SamplerV2.__init__(self, mode, backend, session, options)
    125 if mode is None:
    126     mode = session if backend and session else backend if backend else session
--> 127 BasePrimitiveV2.__init__(self, mode=mode, options=options)

File ~/Qiskit/qiskit-ibm-runtime/qiskit_ibm_runtime/base_primitive.py:106, in BasePrimitiveV2.__init__(self, mode, options)
    104     self._mode = get_cm_session()
    105     self._service = self._mode.service
--> 106     self._backend = self._service.backend(  # type: ignore
    107         name=self._mode.backend(), instance=self._mode._instance
    108     )
    109 else:
    110     self._service = (
    111         QiskitRuntimeService()
    112         if QiskitRuntimeService.global_service is None
    113         else QiskitRuntimeService.global_service
    114     )

AttributeError: 'QiskitRuntimeLocalService' object has no attribute 'backend'

Expected behavior

It should work.

Suggested solutions

Additional Information

• qiskit-ibm-runtime version: bug exists on qiskit-ibm-runtime 0.23.0 and on current main
• Python version:
• Operating system: Linux