Simplify zipformer model repo in Triton

[wd929]

Hi all,

This PR is to support using 3 onnx models for Zipformer in Triton, which simplifies the previous model_repo (which has 2 more onnx models). We can directly use export-onnx.py in icefall without any modifications. Also add automatic script to download pre-trained models, call export-onnx.py, generate model configurations and finally deploy the model in triton server.

Thanks, Wen

[csukuangfj]

Thanks! Is it ready to merge?

[wd929]

Yes. Thank you!

[csukuangfj]

Thanks!