code: def forward(self, x, residual=None, children=None):
      children = [] if children is None else children
      bottom = self.downsample(x) if self.downsample else x
      residual = self.project(bottom) if self.project else bottom
      if self.level_root:
          children.append(bottom)
      x1 = self.tree1(x, residual)
      if self.level == 1:
          x2 = self.tree2(x1)
          x = self.root([x2, x1] + children)
      else:
          children.append(x1)
          x = self.tree2(x1, children=children)
      return x
  
imports: import paddle
  import paddle.nn
  import paddle.nn.functional
  from ppdet.core.workspace import register, serializable
  from ppdet.modeling.layers import ConvNormLayer
  from ..shape_spec import ShapeSpec
label_desc: Use image node_id 2 for calling the Tree obj's underlying member method code with example usage: obj.forward(x, residual, children) and returns: x