Don't re-retrieve backend in session cm

[kt474]

Summary

Reported in slack - in the case where nothing is passed into the primitive inside a session/batch context manager, the backend is retrieved based on the name. This could cause issues if things in the backend target were modified.

backend = service.backend("ibm_torino")
backend.target.add_instruction(IfElseOp, name="if_else")

with Batch(backend):
    sampler = SamplerV2() # this would re-init the backend, losing the IfElseOp instruction
    sampler.options.experimental = {"execution_path" : "gen3-experimental"}
    job = sampler.run([isa_circuit])

Details and comments

Fixes #

[ElePT]

Thanks for the fix. At a first look it looks reasonable, but it would be better if we could have a regression test to confirm that the code works. I also added a reno suggestion that took me way too long to write :)

[ElePT]

LGTM, thanks!