Add ViTPose ONNX export (#2183)
* Add ONNX export support for ViTPose
* building dummy inputs for vit post
* Move vitpose config to custom class
* Move input generators
* Patch VitPose models with num_experts>1
* Formatting
* Add vitpose export unit tests
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Co-authored-by: Joshua Lochner <admin@xenova.com>
