https://ww2.mathworks.cn/help/deeplearning/ref/placeholderlayer.html
PlaceholderLayer - Layer replacing an unsupported Keras or ONNX layer - MATLAB
PlaceholderLayer is a layer that importKerasLayers and importONNXLayers insert into a layer array or layer graph in place of an unsupported Keras or ONNX layer.
layerreplacingunsupportedkerasonnx