title: What's the diff between tf.import_graph_def and tf.train.import_meta_graph
body: <p>When training in model folder there are auto saved meta graph files.<br>
  So what's the diff between graph and meta graph.<br>
  If I want to load model and do inference witout building graph from scratch,<br>
  use tf.train.import_meta_grah is fine?  </p>