fix OpenCV num_threads log

[Sanster]

Correctly print the num_threads of opencv

[CLAassistant]

All committers have signed the CLA.

[MeowZheng]

many thanks for your nice pr, please sign CLA first

[codecov]

Codecov Report

Merging #1704 (840fd63) into master (a8a8fec) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1704   +/-   ##
=======================================
  Coverage   90.24%   90.24%           
=======================================
  Files         142      142           
  Lines        8486     8486           
  Branches     1432     1432           
=======================================
  Hits         7658     7658           
  Misses        588      588           
  Partials      240      240           

Flag	Coverage Δ
unittests	90.24% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmseg/utils/set_env.py	84.37% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 33606ca...840fd63. Read the comment docs.