Add missing match on onnx/model.onnx download

[alvarobartt]

What does this PR do?

This PR adds a missing match statement within the download_onnx function as it was missing for the onnx/model.onnx download, leading to an unexpected logging message claiming that the ONNX files were downloaded successfully, while those were not indeed (see attachment at huggingface/Google-Cloud-Containers#132 (comment)).

This PR fixes a non-breaking issue, as even if the logging messages were misleading the behaviour for rolling back to safetensors if ONNX weights were not available is working as expected.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@OlivierDehaene