Fix supports_{tp/pp}_plan

[hmellor]

In configs, base_model_pp_plan and base_model_tp_plan default to None

In models, _pp_plan and _tp_plan look like they default to None based on the class variables, but will actually always be a dict because of post_init.

This means that supports_pp_plan and supports_tp_plan always return True.

This PR:

• Fixes the supports methods
• Adds some input validation to pp_plan's setter (same as tp_plan's)
• Fixes the type hint for PreTrainedModel._pp_plan because it's always a tuple[str, str] not an Enum

[HuggingFaceDocBuilderDev]

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[Cyrilvallez]

I think we should add a checl for emptyness!

[Cyrilvallez]

Shiuldn't we check if it's empty?

[hmellor]

I was checking for truthiness because the default value is None if post_init is not called

[Cyrilvallez]

Same here, let's check if it's empty

[Cyrilvallez]

Alright, we can use the implicit bool value of the dict

[Cyrilvallez]

@bot /style

[github-actions]

Style fix bot fixed some files and pushed the changes.