how to freeze save_model.pb

def frozen_graph_maker(export_dir,output_graph):
        with tf.Session(graph=tf.Graph()) as sess:
                tf.saved_model.loader.load(sess, [tf.saved_model.tag_constants.SERVING], export_dir)
                output_nodes = [n.name for n in tf.get_default_graph().as_graph_def().node]
                output_graph_def = tf.graph_util.convert_variables_to_constants(
                        sess, # The session is used to retrieve the weights
                        sess.graph_def,
                        output_nodes# The output node names are used to select the usefull nodes
               )       
        # Finally we serialize and dump the output graph to the filesystem
        with tf.gfile.GFile(output_graph, "wb") as f:
                f.write(output_graph_def.SerializeToString())
def main():
        export_dir='/dir/of/pb/and/variables'
        output_graph = "frozen_graph.pb"
        frozen_graph_maker(export_dir,output_graph)