code: def forward(self, feats):
      # feats: [P3 - P7]
      lateral_feats = []
  
      # up
      up_feature = feats[-1]
      for i, feature in enumerate(feats[::-1]):
          if i == 0:
              lateral_feats.append(feature)
          else:
              shape = paddle.shape(feature)
              up_feature = F.interpolate(
                  up_feature, size=[shape[2], shape[3]]
              )
              lateral_feature = self._feature_fusion_cell(
                  self.conv_up[i - 1],
                  feature,
                  up_feature,
                  weights=self.up_weights[i - 1]
                  if self.use_weighted_fusion
                  else None,
              )
              lateral_feats.append(lateral_feature)
              up_feature = lateral_feature
  
      out_feats = []
      # down
      down_feature = lateral_feats[-1]
      for i, (lateral_feature, route_feature) in enumerate(
          zip(lateral_feats[::-1], feats)
      ):
          if i == 0:
              out_feats.append(lateral_feature)
          else:
              down_feature = F.max_pool2d(down_feature, 3, 2, 1)
              if i == len(feats) - 1:
                  route_feature = None
                  weights = (
                      self.down_weights[i - 1][:2]
                      if self.use_weighted_fusion
                      else None
                  )
              else:
                  weights = (
                      self.down_weights[i - 1]
                      if self.use_weighted_fusion
                      else None
                  )
              out_feature = self._feature_fusion_cell(
                  self.conv_down[i - 1],
                  lateral_feature,
                  down_feature,
                  route_feature,
                  weights=weights,
              )
              out_feats.append(out_feature)
              down_feature = out_feature
  
      return out_feats
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from paddle import ParamAttr
  from paddle.nn.initializer import Constant
  from ppdet.core.workspace import register, serializable
  from ppdet.modeling.layers import ConvNormLayer
  from ..shape_spec import ShapeSpec
label_desc: Use image node_id 3 for calling the BiFPNCell obj's underlying member method code with example usage: obj.forward(feats) and returns: out_feats