[Fix] Fix for tmp_folder in OpenCVInit

[dreamerlin]

This PR corrects #354.

[codecov]

Codecov Report

Merging #357 (d283eb7) into master (25b88f1) will decrease coverage by 0.11%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   86.20%   86.09%   -0.12%     
==========================================
  Files          98       98              
  Lines        6924     6926       +2     
  Branches     1119     1120       +1     
==========================================
- Hits         5969     5963       -6     
- Misses        736      742       +6     
- Partials      219      221       +2     

Flag	Coverage Δ
unittests	86.09% <33.33%> (-0.10%)	⬇️

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Impacted Files	Coverage Δ
mmaction/datasets/pipelines/loading.py	85.14% <33.33%> (-0.56%)	⬇️
mmaction/datasets/pipelines/augmentations.py	94.45% <0.00%> (-0.55%)	⬇️

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[innerlee]

it would be great to have a test