Add ONNX export support for ModernBERT

[xenova]

What does this PR do?

This PR adds ONNX export support for ModernBERT, added to transformers in huggingface/transformers#35158. It uses torch.compile decorators for certain functions, which is not supported in torch.onnx.export (with dynamo=False), so I introduced a model loader context manager to disable this when the model is being loaded. Note that this cannot happen in a model patcher since by then, the model class has already been created.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Who can review?

[xenova]

Now works with sentence-transformers & transformers models 👍

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[echarlaix]

LGTM, thanks a lot @xenova