Fix ONNX export script

[xenova]

Converted models can be found at https://huggingface.co/Xenova/yolov9-onnx/tree/main.

closes #2

[895122938]

Thanks！！！

[mdciri]

@xenova, do these model have NMS integrated? if yes, would you share your model export code in order to perform inference with onnxruntime? Thnaks in advance