Get adaptive scales according to areas. The scale range is [0.5, 1.0]. When the area is less than ``'min_area'``, the scale is 0.5 while the area is larger than ``'max_area'``, the scale is 1.0. Args: areas (ndarray): The areas of bboxes or masks with the shape
(areas, min_area=800, max_area=30000)
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