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]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.