code: def forward(self, img1, img2):
      channel = img1.shape[1]
      if channel == self.channel and self.window.dtype == img1.dtype:
          window = self.window
      else:
          window = self.create_window(self.window_size, channel)
          self.window = window
          self.channel = channel
  
      return self._ssim(
          img1,
          img2,
          window,
          self.window_size,
          channel,
          self.size_average,
      )
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import math
  import paddle
  import paddle.nn
  import paddle.nn.functional
  from paddle import ParamAttr
  from ppdet.core.workspace import register
  from ppdet.modeling import ops
  from ppdet.modeling.losses.iou_loss import GIoULoss
  from ppdet.utils.logger import setup_logger
label_desc: Use image node_id 5 for calling the SSIM obj's underlying member method code with example usage: obj.forward(img1, img2) and returns: self