Bump_v0.20.2

[MengzhangLI]

Reference PR: #1138 .

Reason for making this version: fix BC-Breaking of #1138 .

[codecov]

Codecov Report

Merging #1141 (216c62c) into master (bc4835d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1141   +/-   ##
=======================================
  Coverage   90.11%   90.11%           
=======================================
  Files         125      125           
  Lines        7262     7262           
  Branches     1206     1206           
=======================================
  Hits         6544     6544           
  Misses        515      515           
  Partials      203      203           

Flag	Coverage Δ
unittests	90.11% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmseg/version.py	58.33% <100.00%> (ø)

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