code: def get_all_anchors_fpn(strides=None, sizes=None):
      
      if strides is None:
          strides = cfg.FPN.ANCHOR_STRIDES
      if sizes is None:
          sizes = cfg.RPN.ANCHOR_SIZES
      assert len(strides) == len(sizes)
      foas = []
      for stride, size in zip(strides, sizes):
          foa = get_all_anchors(stride=stride, sizes=(size,))
          foas.append(foa)
      return foas
docs: Returns:
          [anchors]: each anchors is a SxSx NUM_ANCHOR_RATIOS x4 array.