Add pipeline parallel plan to `PretrainedConfig` and `PreTrainedModel`
Author: hmellor
Pull request: yes

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Fixes # (issue)


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_36091). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

I've simplified the schema as suggested. Perhaps we could also add an `Enum` to so that users can index these schemas without knowing that `[0]` means inputs and `[1]` means outputs?

Thanks both for the reviews!

Since there is currently no pipeline logic added by this PR let's save testing for a follow up PR.