Add Onnx Config for ImageGPT 
# What does this PR do?
Fixes #16308

Add changes to make ImageGPT models available for Onnx conversion.

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@ChainYo
_The documentation is not available anymore as the PR was closed or merged._<|||||>@lewtun With latest changes, all the 4 tests are now passing.

![image](https://user-images.githubusercontent.com/52318784/198269069-0fa809f5-ae25-4b7e-8276-c34d7a941017.png)
<|||||>> @lewtun With the latest changes, all the 4 tests are now passing.

Thanks for iterating so fast, @RaghavPrabhakar66. Good work!

If you have time, you can try to upload an ONNX ImageGPT model to the ONNX organization on the hub if you want.<|||||>@ChainYo Sure.<|||||>@sgugger fixed it.<|||||>Thanks!