Fix func docstring

[kwonmha]

Fix docstring of function load_adapter.

1. model_id type hint
   add Pathlike to keep consistency with other functions that call load_adapter.

2. add missing model_id docstring

3. remove peft_config docstring as peft_config argument doesn't exist.

[HuggingFaceDocBuilderDev]

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[BenjaminBossan]

Thanks for adjusting the type annotation and fixing the docstring.