Bump version to v2.18.0

[RangiLyu]

This PR bump the version to v2.18.0

[codecov]

Codecov Report

Merging #6365 (efed6fa) into master (9874180) will decrease coverage by 0.18%.
The diff coverage is 45.25%.

@@            Coverage Diff             @@
##           master    #6365      +/-   ##
==========================================
- Coverage   61.80%   61.61%   -0.19%     
==========================================
  Files         315      318       +3     
  Lines       25174    25376     +202     
  Branches     4187     4230      +43     
==========================================
+ Hits        15558    15635      +77     
- Misses       8805     8922     +117     
- Partials      811      819       +8     

Flag	Coverage Δ
unittests	61.59% <45.25%> (-0.19%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/apis/train.py	16.00% <0.00%> (-0.22%)	⬇️
mmdet/models/roi_heads/mask_heads/fcn_mask_head.py	64.73% <0.00%> (-0.38%)	⬇️
mmdet/datasets/builder.py	55.81% <7.69%> (-5.73%)	⬇️
mmdet/models/roi_heads/sparse_roi_head.py	70.06% <18.75%> (-24.17%)	⬇️
mmdet/datasets/samplers/infinite_sampler.py	23.75% <23.75%> (ø)
mmdet/models/dense_heads/anchor_head.py	85.34% <50.00%> (-1.39%)	⬇️
mmdet/models/detectors/queryinst.py	83.33% <83.33%> (ø)
...t/models/roi_heads/mask_heads/dynamic_mask_head.py	83.33% <83.33%> (ø)
mmdet/datasets/pipelines/transforms.py	77.09% <100.00%> (ø)
mmdet/datasets/samplers/__init__.py	100.00% <100.00%> (ø)
... and 10 more

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