Returns a list of the names of the inputs to this model.
    :param spec: The model protobuf specification
    :return: [str] A list of input feature names

def _get_input_names(spec):
    """
    Returns a list of the names of the inputs to this model.
    :param spec: The model protobuf specification
    :return: [str] A list of input feature names
    """
    retval = [feature.name for feature in spec.description.input]
    return retval