code: def _export_signatures(meta_graph):
    
    named_signatures = tf_v1.get_collection(_SIGNATURE_COLLECTION)
    if not named_signatures:
      raise ValueError("No signatures present. Please call hub.add_signature(...)"
                       "at least once in the module_fn.")
    for key, signature in named_signatures:
      meta_graph.signature_def[key].CopyFrom(signature)
docs: Exports signatures from current graph into a MetaGraphDef.