Fix generate_dummy_inputs for ImageGPTOnnxConfig
# What does this PR do?

`ImageGPT` ONNX tests fail with

```bash
TypeError: __call__() takes 2 positional arguments but 3 were given
```

This is due to the way of calling `preprocessor(input_image, framework)` is not working with the changes introduced in Image Process PR #19796.

This PR updates the calling way by using keyword arguments.

_The documentation is not available anymore as the PR was closed or merged._