I transformed the pytorch model to onnx :
  %79 : Tensor = onnx::Unsqueeze[axes=[0]](%75)
  %80 : Tensor = onnx::Unsqueeze[axes=[0]](%77)
  %81 : Tensor = onnx::Concat[axis=0](%78, %79, %80)
  %82 : Float(2, 1, 256) = onnx::ConstantOfShape[value={0}](%81), scope: CRNN/Sequential[rnn]/BidirectionalLSTM[0]/LSTM[rnn]
  %83 : Tensor? = prim::Constant(), scope: CRNN/Sequential[rnn]/BidirectionalLSTM[0]/LSTM[rnn]

Does  %83:prim::Constant()  belong to onnx? I did not find that op in onnx 3.