[Fix]: Fix sampler unit test.

[RangiLyu]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers

Modification

Fix import path incorrect in sampler unit test.
Fix a potential bug in the unit test of the VOC dataset.

BC-breaking (Optional)

None

[codecov]

Codecov Report

Merging #6284 (a8ea29d) into master (c88509c) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6284   +/-   ##
=======================================
  Coverage   61.80%   61.80%           
=======================================
  Files         315      315           
  Lines       25157    25163    +6     
  Branches     4185     4185           
=======================================
+ Hits        15549    15553    +4     
  Misses       8798     8798           
- Partials      810      812    +2     

Flag	Coverage Δ
unittests	61.78% <ø> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmdet/core/bbox/samplers/random_sampler.py	75.00% <0.00%> (-5.56%)	⬇️
mmdet/datasets/pipelines/transforms.py	76.91% <0.00%> (-0.18%)	⬇️
mmdet/models/detectors/two_stage.py	68.23% <0.00%> (ø)
mmdet/models/roi_heads/standard_roi_head.py	62.11% <0.00%> (ø)
mmdet/models/dense_heads/yolact_head.py	79.55% <0.00%> (+0.17%)	⬆️
mmdet/core/bbox/assigners/max_iou_assigner.py	73.68% <0.00%> (+1.31%)	⬆️
mmdet/models/detectors/yolact.py	97.36% <0.00%> (+3.08%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c88509c...a8ea29d. Read the comment docs.