[enhance]: Support batch_size > 1 in validation (#3852)

[RunningLeon]

Modify to support batch_size > 1 in validation mode.
Resolves #3852

[codecov]

Codecov Report

Merging #3966 into master will increase coverage by 0.04%.
The diff coverage is 25.00%.

@@            Coverage Diff             @@
##           master    #3966      +/-   ##
==========================================
+ Coverage   62.27%   62.32%   +0.04%     
==========================================
  Files         221      221              
  Lines       16311    16314       +3     
  Branches     2758     2759       +1     
==========================================
+ Hits        10158    10167       +9     
+ Misses       5660     5654       -6     
  Partials      493      493              

Flag	Coverage Δ
#unittests	62.32% <25.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
mmdet/apis/train.py	16.66% <25.00%> (-0.80%)	⬇️
mmdet/models/dense_heads/corner_head.py	76.77% <0.00%> (+1.91%)	⬆️
mmdet/models/detectors/cornernet.py	100.00% <0.00%> (+5.12%)	⬆️

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[hellock]

Task linked: CU-4qrz4e Support batch_size > 1 in validation mode (#3852)