[Docs] Add test_mim.yml for testing commands of mim in CI

[Zachary-66]

Test commands of mim in CI, related pr open-mmlab/mmsegmentation#1654

[codecov]

Codecov Report

Merging #158 (fc7a003) into master (d8e7e81) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
+ Coverage   68.82%   68.88%   +0.05%     
==========================================
  Files          92       92              
  Lines        5148     5148              
  Branches      733      733              
==========================================
+ Hits         3543     3546       +3     
+ Misses       1491     1488       -3     
  Partials      114      114              

Flag	Coverage Δ
unittests	68.88% <ø> (+0.05%)	⬆️

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Impacted Files	Coverage Δ
mmflow/datasets/pipelines/transforms.py	79.83% <0.00%> (+0.50%)	⬆️

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