Fix ImageGPT doctest
# What does this PR do?

#24317 Resolved the ImageGPT doc test failing issue, as `clusters` in the image processor were not stored as numpy arrays as expected. This was tested by running the code directly, but I didn't run using  
` pytest --doctest-modules src/transformers/models/imagegpt/modeling_imagegpt.py ` 🙃 

The tests were failing because some code produces an output e.g. model architecture when caling `model.to`, but no "expected" output is provided. We don't want to check these outputs, so this PR adds controls to ignore. 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?

_The documentation is not available anymore as the PR was closed or merged._<|||||>@ydshieh Running the doctests (properly this time :) ), the tests pass with the ignore statement on the for loop, and fail without (in the same way as on the CI). 