Add Onnx Config for ImageGPT (#19868)
* add Onnx Config for ImageGPT
* add generate_dummy_inputs for onnx config
* add TYPE_CHECKING clause
* Update doc for generate_dummy_inputs
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
