Add ONNX export support for IJepa

[xenova]

What does this PR do?

Added in huggingface/transformers#33125

TODO: Create and add tiny-random models

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?

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[HuggingFaceDocBuilderDev]

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