Fix bug in non-hermitian gradient

Created by: PhilipVinc

In the refactor prior to chunking I forgot to update the code for non hermitian operators. I did not notice because the test is marked as expect-fail (it would probably work if we generalise the finite difference gradient we use for tests, as I used it several times and results seem correct) and so I never noticed the code did not run anymore.

this fixes the bug and adds a test that checks that the code runs without checking correctness.