[WIP] Add onnx export support for CLIPSeg models

[xenova]

What does this PR do?

Two things need to be updated in transformers:

1. Fix casting here, here, and here
2. Add the following line here:

   ("clipseg", "CLIPSegForImageSegmentation"),
   

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]

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