* 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>