code: def ssim(self, img1, img2, window_size=11, size_average=True):
      (_, channel, _, _) = img1.shape
      window = self.create_window(window_size, channel)
  
      return self._ssim(
          img1, img2, window, window_size, channel, size_average
      )
  
imports: from math import exp
  import paddle
  import paddle.nn.functional
  import paddle.nn
  import string
label_desc: Use image node_id 5 for calling the SSIM obj's underlying member method code with example usage: obj.ssim(img1, img2, window_size, size_average) and returns: self