Fix ImageGPT doctest

[amyeroberts]

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.

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[ydshieh]

Thanks @amyeroberts . I don't know we can add ignore at the for for loop line, but I believe you check that is the correct way to do :-)

[HuggingFaceDocBuilderDev]

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

[amyeroberts]

@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).