Update `OnnxConfig.generate_dummy_inputs` to check `ImageProcessingMixin`
# What does this PR do?

As we have a new class `ImageProcessingMixin`, the method `OnnxConfig.generate_dummy_inputs` needs to check this case, otherwise it can't find a if/else branch to create dummy inputs.

See 
https://github.com/huggingface/transformers/blob/7ec1dc8817a99d16e6f9e0ab94ce4027ef74b72d/src/transformers/onnx/config.py#L371

Current failing error is:

```bash
AssertionError: beit, default -> Unable to generate dummy inputs for the model. Please provide a tokenizer or a preprocessor.
```

_The documentation is not available anymore as the PR was closed or merged._<|||||>We still have `OwlViTFeatureExtractor` and no `OwlViTImageProcessor`.

@amyeroberts I guess it is a miss, right? If so, could you work on adding `OwlViTImageProcessor` in another PR, thanks.<|||||>@ydshieh Yes - the `OwlViTImageProcessor` doesn't exist yet (along with other object detection / segmentation models). These will be added soon. There's two pending PRs:
* Adding transforms: https://github.com/huggingface/transformers/pull/20003
* Adding these models image processors: https://github.com/huggingface/transformers/pull/20160

I believe this should be only affect OwlViT - as it has a processor class which contains both the feature extractor and the image processor. As you've done in the PR - I think maintaining a check for both `FeatureExtactionMixin` and `ImageProcessingMixin` should work. We can then remove the check for 
`elif isinstance(preprocessor, FeatureExtractionMixin) and preprocessor.model_input_names[0] == "pixel_values":` 
