code: def __init__(self, window_size, num_heads):
      super().__init__()
      self.window_size = window_size
      self.num_relative_distance = (2 * window_size[0] - 1) * (
          2 * window_size[1] - 1
      ) + 3
      self.relative_position_bias_table = self.create_parameter(
          shape=(self.num_relative_distance, num_heads),
          default_initialize=zeros_,
      )
      # cls to token & token 2 cls & cls to cls
  
      # get pair-wise relative position index for each token inside the window
      coords_h = paddle.arange(window_size[0])
      coords_w = paddle.arange(window_size[1])
      coords = paddle.stack(
          paddle.meshgrid([coords_h, coords_w])
      )  # 2, Wh, Ww
      coords_flatten = coords.flatten(1)  # 2, Wh*Ww
  
      relative_coords = (
          coords_flatten[:, :, None] - coords_flatten[:, None, :]
      )  # 2, Wh*Ww, Wh*Ww
      relative_coords = relative_coords.transpos(
          (1, 2, 0)
      )  # Wh*Ww, Wh*Ww, 2
      relative_coords[:, :, 0] += (
          window_size[0] - 1
      )  # shift to start from 0
      relative_coords[:, :, 1] += window_size[1] - 1
      relative_coords[:, :, 0] *= 2 * window_size[1] - 1
      relative_position_index = paddle.zeros(
          size=(window_size[0] * window_size[1] + 1,) * 2,
          dtype=relative_coords.dtype,
      )
      relative_position_index[1:, 1:] = relative_coords.sum(
          -1
      )  # Wh*Ww, Wh*Ww
      relative_position_index[0, 0:] = self.num_relative_distance - 3
      relative_position_index[0:, 0] = self.num_relative_distance - 2
      relative_position_index[0, 0] = self.num_relative_distance - 1
      self.register_buffer(
          "relative_position_index", relative_position_index
      )
  
imports: import math
  import paddle
  import paddle.nn
  import paddle.nn.functional
  import numpy
  from paddle.nn.initializer import Constant
  from ppdet.modeling.shape_spec import ShapeSpec
  from ppdet.core.workspace import register, serializable
  from .transformer_utils import zeros_, DropPath, Identity
label_desc: Use image node_id 1 to create a new RelativePositionBias object from inherited base classes: nn with example: obj = RelativePositionBias(window_size, num_heads)