Support MIM

[AronLin]

• add MIM introduction in README
• modify setup.py to support mim as mmdet entry point

[codecov]

Codecov Report

Merging #5676 (05a6050) into master (9921ee3) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 05a6050 differs from pull request most recent head 1419e93. Consider uploading reports for the commit 1419e93 to get more accurate results

@@           Coverage Diff           @@
##           master    #5676   +/-   ##
=======================================
  Coverage   66.38%   66.39%           
=======================================
  Files         279      279           
  Lines       21962    21962           
  Branches     3650     3650           
=======================================
+ Hits        14580    14581    +1     
  Misses       6633     6633           
+ Partials      749      748    -1     

Flag	Coverage Δ
unittests	66.36% <ø> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmdet/models/losses/ghm_loss.py	93.81% <0.00%> (+1.03%)	⬆️

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