如何将tensorflow saved_model转换为ckpt？

import tensorflow as tf
from tensorflow.python.tools import freeze_graph
from tensorflow.python.tools import optimize_for_inference_lib

tf.reset_default_graph()
sess = tf.Session()
meta_graph_def = tf.saved_model.loader.load(sess, [tf.saved_model.tag_constants.SERVING], 'path_to_saved_model')

graph_def = tf.get_default_graph().as_graph_def()

output_node_names = 'output_node_name'  # 替换为模型输出节点的名称
output_graph_def = freeze_graph.freeze_graph(None, None, None, None, None, None, None, 'frozen_graph.pb', False, '', 'save/restore_all', 'save/Const:0', '', '')

input_graph_def = tf.GraphDef()
with tf.gfile.Open('frozen_graph.pb', 'rb') as f:
    data = f.read()
    input_graph_def.ParseFromString(data)

output_graph_def = optimize_for_inference_lib.optimize_for_inference(input_graph_def, ['input_node_name'], ['output_node_name'], tf.float32.as_datatype_enum)

tf.train.write_graph(output_graph_def, '.', 'model.ckpt', as_text=False)