code: public com.google.protobuf.ByteString
        getSessionHandleBytes() {
      java.lang.Object ref = sessionHandle_;
      if (ref instanceof java.lang.String) {
        com.google.protobuf.ByteString b = 
            com.google.protobuf.ByteString.copyFromUtf8(
                (java.lang.String) ref);
        sessionHandle_ = b;
        return b;
      } else {
        return (com.google.protobuf.ByteString) ref;
      }
    }
docs: <pre>
  Subgraphs are scoped within one session.
  </pre>
  
  <code>optional string session_handle = 1;</code>