Bug in SSIM: Can't reproduce paper values

**Describe the bug**
Performance of SSIM on TID2013 dataset:
PIQ - SRCC **0.5544**  KRCC **0.3883**
Original paper - SRCC **0.637** KRCC **0.464**
DISTS paper - SRCC **0.727** KRCC **0.545**

**To Reproduce**
Code is not available. Will be released soon (see #207).

**Expected behavior**
Match performance to original paper
