Add support for DiT ONNX export

[xenova]

What does this PR do?

Not sure if this is actually needed, since DiT uses the same architecture as BeiT, but it adds it to the list 👍

Fixes # (issue)

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?

[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.

[fxmarty]

@xenova

Can you fix

FAILED exporters/onnx/test_onnx_export.py::OnnxExportTestCase::test_all_models_tested - AssertionError: Not testing all models. Missing models: {'dit'}
2023-12-24T18:41:32.3343756Z FAILED onnxruntime/test_modeling.py::ORTModelForImageClassificationIntegrationTest::test_pipeline_ort_model_01_dit - KeyError: 'dit'
2023-12-24T18:41:32.3344416Z FAILED onnxruntime/test_modeling.py::ORTModelForImageClassificationIntegrationTest::test_compare_to_transformers_01_dit - KeyError: 'dit'

[xenova]

Converted to draft and will mark as ready when I've added back the unit tests. @fxmarty once again, it requires a tiny model - do you have a script to easily create these?