bump version to 1.2.0

[ycxioooong]

No description provided.

[codecov]

Codecov Report

Merging #644 (ebce383) into master (bb06f35) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #644   +/-   ##
=======================================
  Coverage   65.04%   65.04%           
=======================================
  Files         134      134           
  Lines        7730     7730           
  Branches     1370     1370           
=======================================
  Hits         5028     5028           
  Misses       2439     2439           
  Partials      263      263           

Flag	Coverage Δ
unittests	65.04% <100.00%> (ø)

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

Impacted Files	Coverage Δ
mmcv/version.py	92.30% <100.00%> (ø)

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