class_docstr: Compute rotated anchors used by Rotated RPN (RRPN), described in
  "Arbitrary-Oriented Scene Text Detection via Rotation Proposals".
code: def _calculate_anchors(self, sizes, aspect_ratios, angles):
      cell_anchors = [
          self.generate_cell_anchors(size, aspect_ratio, angle).float()
          for size, aspect_ratio, angle in zip(
              sizes, aspect_ratios, angles
          )
      ]
      return BufferList(cell_anchors)
  
imports: import collections
  import math
  from typing import List
  import torch
  from torch import nn
  from detectron2.config import configurable
  from detectron2.layers import ShapeSpec, move_device_like
  from detectron2.structures import Boxes, RotatedBoxes
  from detectron2.utils.registry import Registry
label_desc: Use image node_id 3 for calling the RotatedAnchorGenerator obj's underlying member method code with example usage: obj._calculate_anchors(sizes, aspect_ratios, angles) and returns: BufferList