Returns CustomModels for models with a custom `__implementation__`

def _get_custom_models(models):
    """Returns CustomModels for models with a custom `__implementation__`"""
    if models is None:
        models = Model.model_class_reverse_map.values()

    custom_models = OrderedDict()
    for cls in models:
        impl = getattr(cls, "__implementation__", None)

        if impl is not None:
            model = CustomModel(cls)
            custom_models[model.full_name] = model

    if not custom_models:
        return None
    return custom_models