VisualizationHook: check channels before bgr2rgb

[zeakey]

bgr2rgb conversion in VisualizationHook results in CUDA error for single channel images. I added a channels==3 check for protection.

[codecov]

Codecov Report

Base: 74.81% // Head: 74.81% // No change to project coverage 👍

Coverage data is based on head (2eb52f5) compared to base (8d0925c).
Patch coverage: 0.00% of modified lines in pull request are covered.

❗ Current head 2eb52f5 differs from pull request most recent head 4383b6b. Consider uploading reports for the commit 4383b6b to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #495   +/-   ##
=======================================
  Coverage   74.81%   74.81%           
=======================================
  Files         145      145           
  Lines       11015    11015           
  Branches     2097     2097           
=======================================
  Hits         8241     8241           
  Misses       2233     2233           
  Partials      541      541           

Flag	Coverage Δ
unittests	74.81% <0.00%> (ø)

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Impacted Files	Coverage Δ
mmgen/core/hooks/visualization.py	91.17% <0.00%> (ø)

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