onnx LSTM model failed loading

[hengdaidi]

onnx LSTM model failed loading into RKNN. Model in the below link could reproduce the issue.
https://raw.githubusercontent.com/microsoft/onnxruntime/master/onnxruntime/test/testdata/CNTK/test_LSTM.tanh.bidirectional/model.onnx

E Catch exception when loading onnx model: ../models/model.onnx!
E Traceback (most recent call last):
E File "rknn/api/rknn_base.py", line 510, in rknn.api.rknn_base.RKNNBase.load_onnx
E File "rknn/base/RKNNlib/converter/convert_onnx.py", line 514, in rknn.base.RKNNlib.converter.convert_onnx.convert_onnx.init
E File "rknn/base/RKNNlib/converter/convert_onnx.py", line 176, in rknn.base.RKNNlib.converter.convert_onnx.onnx_shape_infer_engine.infer_shape
E File "rknn/base/RKNNlib/converter/convert_onnx.py", line 131, in rknn.base.RKNNlib.converter.convert_onnx.onnx_shape_infer_engine.__status_update
E File "rknn/base/RKNNlib/converter/convert_onnx.py", line 165, in rknn.base.RKNNlib.converter.convert_onnx.onnx_shape_infer_engine.__model_rebuild
E File "rknn/base/RKNNlib/converter/tensor_model.py", line 330, in rknn.base.RKNNlib.converter.tensor_model.TensorModel.build_onnx_model
E KeyError: ''
Load failed!