code: def forward(self, f1, f2, f3, f4):
      # up-down
      f3 = self.smooth_layer3_1(
          self.dwconv3_1(self._upsample_add(f4, f3))
      )
      f2 = self.smooth_layer2_1(
          self.dwconv2_1(self._upsample_add(f3, f2))
      )
      f1 = self.smooth_layer1_1(
          self.dwconv1_1(self._upsample_add(f2, f1))
      )
  
      # down-up
      f2 = self.smooth_layer2_2(
          self.dwconv2_2(self._upsample_add(f2, f1))
      )
      f3 = self.smooth_layer3_2(
          self.dwconv3_2(self._upsample_add(f3, f2))
      )
      f4 = self.smooth_layer4_2(
          self.dwconv4_2(self._upsample_add(f4, f3))
      )
  
      return f1, f2, f3, f4
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import paddle
  from paddle import nn
  import paddle.nn.functional
  from paddle import ParamAttr
  import os
  import sys
  import math
  from paddle.nn.initializer import TruncatedNormal, Constant, Normal
label_desc: Use image node_id 3 for calling the FPEM obj's underlying member method code with example usage: obj.forward(f1, f2, f3, f4) and returns: f1,  f2,  f3,  f4