Dear li, peng
Though OpenVino 2019R2 has been released the documentation is lagging a week behind. It should be fully updated in a week or so. But I checked in our R2 code and I saw this :
C:\Program Files (x86)\IntelSWTools\openvino_2019.2.242\deployment_tools\model_optimizer\extensions\front\onnx\slice_ext.py
Which definitely leads me to believe that the slice op is supported by Model Optimizer ONNX. Have you tried R2 yet ?