Add mim to extras_require in setup.py

[MeowZheng]

ref open-mmlab/mmsegmentation#1655

[codecov]

Codecov Report

Merging #153 (381ec1f) into master (8aee672) will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #153      +/-   ##
==========================================
- Coverage   68.82%   68.72%   -0.10%     
==========================================
  Files          92       92              
  Lines        5148     5148              
  Branches      733      733              
==========================================
- Hits         3543     3538       -5     
- Misses       1492     1495       +3     
- Partials      113      115       +2     

Flag	Coverage Δ
unittests	68.72% <ø> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
mmflow/datasets/pipelines/transforms.py	78.66% <0.00%> (-0.84%)	⬇️
mmflow/datasets/dataset_wrappers.py	43.75% <0.00%> (ø)

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