code: def forward(self, img1, img2):
      (_, channel, _, _) = img1.shape
  
      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 math import exp
  import paddle
  import paddle.nn.functional
  import paddle.nn
  import string
label_desc: Use image node_id 6 for calling the SSIM obj's underlying member method code with example usage: obj.forward(img1, img2) and returns: self