Register a new model that can be obtained with `get_model_config`.

def register_model(model_name, dataset_name, model_func):
    """Register a new model that can be obtained with `get_model_config`."""
    model_map = _get_model_map(dataset_name)
    if model_name in model_map:
        raise ValueError("Model \"%s\" is already registered for dataset"
                         "\"%s\"" % (model_name, dataset_name))
    model_map[model_name] = model_func