def save_summaries(file_writer, global_step=None):
    """Call FileWriter.add_summary() with all summaries in the default graph,
    automatically finalizing and merging them on the first call.
    """
    global _merge_op
    tfutil.assert_tf_initialized()

    if _merge_op is None:
        layout = finalize_autosummaries()
        if layout is not None:
            file_writer.add_summary(layout)
        with tf.device(None), tf.control_dependencies(None):
            _merge_op = tf.summary.merge_all()

    file_writer.add_summary(_merge_op.eval(), global_step)
Call FileWriter.add_summary() with all summaries in the default graph,
    automatically finalizing and merging them on the first call.
    
converters/stylegan2_official/dnnlib/tflib/autosummary.py