onnx should be 1.6.0

for smoothly export model to onnx, onnx should be 1.6.0

In torch==1.5, the import of the API has changed. Although it does not interrupt the operation of the program, it seems to me to be an implicit error and may throw an exception in later versions.

Delete redundant thrid party library

I submitted it once in your yolov3 project, you seem to accept it? I'm not sure. I'll submit PR again.

fix reference problem