[Enhancement]: Speed up roi extractor for onnx

[RunningLeon]

This PR speeds up roi extractor for onnx.

[codecov]

Codecov Report

Merging #5194 (d718700) into master (abe9d12) will increase coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #5194      +/-   ##
==========================================
+ Coverage   65.17%   65.21%   +0.03%     
==========================================
  Files         269      269              
  Lines       20844    20847       +3     
  Branches     3500     3500              
==========================================
+ Hits        13586    13595       +9     
+ Misses       6492     6483       -9     
- Partials      766      769       +3     

Flag	Coverage Δ
unittests	65.17% <0.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
...heads/roi_extractors/single_level_roi_extractor.py	65.38% <0.00%> (-4.01%)	⬇️
mmdet/models/dense_heads/corner_head.py	72.60% <0.00%> (+1.86%)	⬆️
mmdet/models/detectors/cornernet.py	100.00% <0.00%> (+5.12%)	⬆️

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