Fix PSNR when input is uint8

[Yshuo-Li]

Fix #288

[codecov]

Codecov Report

Merging #294 (075ee60) into master (3cfdd8f) will not change coverage.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #294   +/-   ##
=======================================
  Coverage   80.54%   80.54%           
=======================================
  Files         162      162           
  Lines        8091     8091           
  Branches     1192     1192           
=======================================
  Hits         6517     6517           
  Misses       1429     1429           
  Partials      145      145           

Flag	Coverage Δ
unittests	80.54% <66.66%> (ø)

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Impacted Files	Coverage Δ
mmedit/core/evaluation/metrics.py	94.88% <66.66%> (ø)

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