Bump version to v1.2.6

[ycxioooong]

No description provided.

[codecov]

Codecov Report

Merging #799 (065332b) into master (00658e8) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #799   +/-   ##
=======================================
  Coverage   61.94%   61.94%           
=======================================
  Files         144      144           
  Lines        8467     8467           
  Branches     1520     1520           
=======================================
  Hits         5245     5245           
  Misses       2954     2954           
  Partials      268      268           

Flag	Coverage Δ
unittests	61.94% <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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