Testing different backends

[stavros11]

Currently some tests are executing circuits using the circuit.execute()/circuit() interface, instead of backend.execute_circuit. This invokes the GlobalBackend which is set to an arbitrary value, potentially from a previous test, unless set_backend is used. More details in https://github.com/qiboteam/qibo/pull/1272#discussion_r1655006022.

A potential solution is to never use circuit() in tests and replace by backend.execute_circuit, except from very few tests that test the circuit() interface itself.

Another solution is to switch the global backend within the backend fixture, using for example something like

@pytest.fixture
def backend(backend_name):
    original_backend = qibo.get_backend()
    qibo.set_backend(backend_name)
    yield
    qibo.set_backend(original_backend)

List of tests that execute circuits using the circuit.execute interface (which invokes the GlobalBackend()), found by inserting an assert False in circuit.execute:

• test_backends_global::test_circuit_execution (sets to numpy)
• test_measurements::test_measurement_compiled_circuit (does not set)
• test_models_circuit_execution::test_compiled_execute (does not set)
• Various tests in test_models_qcnn are using the GlobalBackend but setting it beforehand to qibojit or numpy without reseting it after.
• test_models_variational::test_vqe (does not set)
• test_models_variational::test_custom_loss (does not set)