[Enhancement] Test mim in CI

[gaotongxiao]

Ref: open-mmlab/mmsegmentation#1654

[codecov]

Codecov Report

Merging #1090 (9768c84) into main (d068370) will not change coverage.
The diff coverage is n/a.

❗ Current head 9768c84 differs from pull request most recent head d6a6bf5. Consider uploading reports for the commit d6a6bf5 to get more accurate results

@@           Coverage Diff           @@
##             main    #1090   +/-   ##
=======================================
  Coverage   83.76%   83.76%           
=======================================
  Files         170      170           
  Lines       11372    11372           
  Branches     1750     1750           
=======================================
  Hits         9526     9526           
  Misses       1468     1468           
  Partials      378      378           

Flag	Coverage Δ
unittests	83.76% <ø> (ø)

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