Keras Improved Backend Checking

Before :

requires_trainable_backend = pytest.mark.skipif( backend() == "numpy" or backend() == "openvino", reason="Trainer not implemented for NumPy and OpenVINO backend.", )

After :

requires_trainable_backend = pytest.mark.skipif( backend() in ["numpy", "openvino"], reason="Trainer not implemented for NumPy and OpenVINO backend.", )