[design] adding a `validation` decorator

We try to avoid using the `kwargs` argument at diffusers because it's error prone and makes our code less readable. However, there are some instances where a fixed signature might not be practical. For example, our default attention processor class `AttnProcessor/AttnProcessor2.0` can now be used in conjunction with other attention processors such as `IPAdapterProcessor`. If we need to pass a new argument to `IPAdapterProcessor`, we will need to add it to the `AttnProcessor`'s signature as well. It may be worthwhile to consider adding a `validation` decorator https://github.com/huggingface/huggingface_hub/blob/c528f78fa7404869f32a2b04eac0f9964a5c6f9e/src/huggingface_hub/utils/_validators.py#L46 for this use case

(Original discussion can be found here: https://github.com/huggingface/diffusers/pull/6573#discussion_r1463072069.)