Fix bug in distributed sampler

[LXXXXR]

No description provided.

[codecov]

Codecov Report

Merging #4257 (3ea7d2a) into master (bec3fc1) will increase coverage by 0.00%.
The diff coverage is 80.00%.

@@           Coverage Diff           @@
##           master    #4257   +/-   ##
=======================================
  Coverage   62.69%   62.70%           
=======================================
  Files         232      232           
  Lines       17603    17607    +4     
  Branches     2945     2945           
=======================================
+ Hits        11037    11041    +4     
  Misses       6027     6027           
  Partials      539      539           

Flag	Coverage Δ
unittests	62.66% <80.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmdet/models/dense_heads/yolo_head.py	34.04% <ø> (ø)
mmdet/datasets/samplers/distributed_sampler.py	33.33% <50.00%> (+3.92%)	⬆️
mmdet/models/necks/fpn.py	94.04% <100.00%> (+0.22%)	⬆️

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[hellock]

@ZwwWayne can have a double check.