Fix ImageGPT doc example
# What does this PR do?

There was a bug in the example, where the clusters in the image processor were stored as lists, but the example assumed they were numpy arrays. 

At the moment, clusters are stored as a list of lists, but [converted to a numpy array during processing](https://github.com/huggingface/transformers/blob/0b7b4429c78de68acaf72224eb6dae43616d820c/src/transformers/models/imagegpt/image_processing_imagegpt.py#L230). This PR converts to a numpy array when setting the class attribute and if new `cluster` are passed into `preprocess`. 

This: 
* Maintains backwards compatibility with old configurations
* Saved configs aren't changed (`clusters` is still converted to a list of lists when serializing) see this [dummy image processor](https://huggingface.co/amyeroberts/dummy_imagegpt_image_processor_np_clusters) created from this branch. 
* Is more efficient - we're not converting the same list of lists every batch. 

A potential breaking change is if users were accessing the `clusters` attribute and using it as a list of lists. As this was caught because users were using the clusters as a numpy array (according to the example) I expect this impact to be low. 

Fixes #24189

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- [x] Did you write any new necessary tests?

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