[FEAT] Refactor CUSTOM_ARCHITECTURES to provide custom model

[HarryTheWulf]

This PR will allow for a custom model to be provided for any of the custom architectures - providing null will default to PreTrainedModel.

This came about when I was creating a new model; I wanted to set it up as an Image Segmentation model.

Below shows how a new architecture can be provided with a specific model, rather than PreTrainedModel.

/** @type {CustomArchitectures} */
const CUSTOM_ARCHITECTURES = new Map([
    ['modnet', [MODEL_FOR_IMAGE_SEGMENTATION_MAPPING_NAMES, null]],
    ['birefnet', [MODEL_FOR_IMAGE_SEGMENTATION_MAPPING_NAMES, null]],
    ['isnet', [MODEL_FOR_IMAGE_SEGMENTATION_MAPPING_NAMES, null]],
    ['u2net', [MODEL_FOR_IMAGE_SEGMENTATION_MAPPING_NAMES, U2NetModel]],
    ['ben', [MODEL_FOR_IMAGE_SEGMENTATION_MAPPING_NAMES, null]],
]);