code: def extract_metadata(graph_def):
    
    meta_matches = [n for n in graph_def.node if n.name==metadata_node_name]
    if meta_matches:
      assert len(meta_matches) == 1, "found more than 1 lucid metadata node!"
      meta_tensor = meta_matches[0].attr['value'].tensor
      return json.loads(meta_tensor.string_val[0])
    else:
      return None
docs: Attempt to extract meta data hidden in graph_def.
  
    Looks for a `__lucid_metadata_json` constant string op.
    If present, extract it's content and convert it from json to python.
    If not, returns None.