Fix typos BigBird ONNX conversion

[chainyo]

What does this PR do?

I tried to convert one BigBird model to ONNX with the recent PR merged #16427

But it seems that there is a typo in the src/transformers/onnx/features.py file.
I also fixed the typo in the test_v2 file.

Here is the error I get while trying to convert google/bigbird-roberta-base.

$ python -m transformers.onnx --model=google/bigbird-roberta-base onnx/

> KeyError: "big-bird is not supported yet. 
> Only ['albert', 'bart', 'mbart', 'bert', 'bigbird', 
> 'ibert', 'camembert', 'distilbert', 'flaubert', 'marian', 'm2m-100', 'roberta', 't5', 
> 'xlm-roberta', 'gpt2', 'gptj', 'gpt-neo', 'layoutlm', 'electra', 'vit', 'beit', 'blenderbot', 
> 'blenderbot-small', 'data2vec-text'] are supported. 
> If you want to support big-bird please propose a PR or open up an issue."

As you can see in the error bigbird should be big-bird.

ping @lewtun @LysandreJik

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[lewtun]

Thanks for the fix! This error is caused by the fact that the model_type field of BigBird checkpoints is big_bird, which is mapped to big-bird during the ONNX conversion.