code: def build_ab(self):
      # self.ab = self.attention_biases[:, self.attention_bias_idxs]
      idxs = self.attention_bias_idxs.reshape([-1]).tolist()
      ab = []
      for i in range(self.attention_biases.shape[0]):
          temp = self.attention_biases[i]
          ab.append(temp[idxs])
      self.ab = paddle.concat(ab)
      self.ab = self.ab.reshape((-1, *self.attention_bias_idxs.shape))
  
imports: import itertools
  import paddle
  from paddle import nn
  from paddle.nn import functional
  from .common import LayerNorm2d
label_desc: Use image node_id 2 for calling the Attention obj's underlying member method code with example usage: obj.build_ab() without return types