title: Convert frozen model(.pb) to savedmodel
body: <p>Recently I tried to convert the model (tf1.x) to the saved_model, and followed the official <a href="https://www.tensorflow.org/guide/migrate" rel="noreferrer">migrate document</a>. However in my use case, most of model in my hand or tensorflow model zoo usually is pb file, and according to the <a href="https://www.tensorflow.org/guide/migrate#a_graphpb_or_graphpbtxt" rel="noreferrer">official document</a> says that </p>
  
  <blockquote>
    <p>There is no straightforward way to upgrade a raw Graph.pb file to TensorFlow 2.0, but if you have a "Frozen graph" (a tf.Graph where the variables have been turned into constants), then it is possible to convert this to a concrete_function using v1.wrap_function:</p>
  </blockquote>
  
  <p>But I still do not understand how to converted to <a href="https://www.tensorflow.org/guide/saved_model" rel="noreferrer">saved_model format</a>.</p>