message: training: ssim: improved numerical stability
diff: @@ -66,8 +66,8 @@ def _ssim_per_channel(X, Y, win, data_range=255):
  mu1 = gaussian_filter(X, win)
  mu2 = gaussian_filter(Y, win)
  - mu1_sq = mu1.pow(2)
  - mu2_sq = mu2.pow(2)
  + mu1_sq = mu1 * mu1
  + mu2_sq = mu2 * mu2
  mu1_mu2 = mu1 * mu2
  sigma1_sq = compensation * ( gaussian_filter(X * X, win) - mu1_sq )
  