docs: def annToRLE(self, ann):
          """
          Convert annotation which can be polygons, uncompressed RLE to RLE.
          :return: binary mask (numpy 2D array)
          """
          t = self.imgs[ann['image_id']]
          h, w = t['height'], t['width']
          segm = ann['segmentation']
          if type(segm) == list:
              # polygon -- a single object might consist of multiple parts
              # we merge all parts into one mask rle code
              rles = maskUtils.frPyObjects(segm, h, w)
              rle = maskUtils.merge(rles)
          elif type(segm['counts']) == list:
              # uncompressed RLE
              rle = maskUtils.frPyObjects(segm, h, w)
          else:
              # rle
              rle = ann['segmentation']
          return rle