return a set of strings

def _read_checkpoint_vars(model_path):
        """ return a set of strings """
        reader = tf.train.NewCheckpointReader(model_path)
        reader = CheckpointReaderAdapter(reader)    # use an adapter to standardize the name
        ckpt_vars = reader.get_variable_to_shape_map().keys()
        return reader, set(ckpt_vars)