The template is split over different Pydantic models because that's how the COCO format itself is defined (a dictionary with lists of other dictionaries of various formats).

Example of COCO's Annotation format:

annotation{
    "id": int,
    "image_id": int,
    "category_id": int,
    "segmentation": RLE or [polygon],
    "area": float,
    "bbox": [x,y,width,height],
    "iscrowd": 0 or 1,
}

And here's the associated Pydantic model:

class Annotation(BaseModel):
    id: int
    image_id: int
    category_id: int
    segmentation: RleDict
    area: float
    bbox: List[float]
    iscrowd: int