bump version to v1.2.3

[ycxioooong]

No description provided.

[codecov]

Codecov Report

Merging #761 (cada698) into master (8b4e5de) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #761      +/-   ##
==========================================
- Coverage   64.92%   64.87%   -0.05%     
==========================================
  Files         136      136              
  Lines        7974     7994      +20     
  Branches     1410     1411       +1     
==========================================
+ Hits         5177     5186       +9     
- Misses       2528     2539      +11     
  Partials      269      269              

Flag	Coverage Δ
unittests	64.87% <100.00%> (-0.05%)	⬇️

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)
mmcv/ops/info.py	47.61% <0.00%> (-6.23%)	⬇️
mmcv/ops/nms.py	34.43% <0.00%> (-0.07%)	⬇️
mmcv/ops/__init__.py	100.00% <0.00%> (ø)
mmcv/ops/box_iou_rotated.py	100.00% <0.00%> (ø)

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