Imresize backend

[Johnson-Wang]

Enable flexible backend for imresize

[codecov]

Codecov Report

Merging #3392 into master will decrease coverage by 0.09%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #3392      +/-   ##
==========================================
- Coverage   57.97%   57.88%   -0.10%     
==========================================
  Files         202      202              
  Lines       13491    13493       +2     
  Branches     2282     2282              
==========================================
- Hits         7822     7811      -11     
- Misses       5310     5322      +12     
- Partials      359      360       +1     

Flag	Coverage Δ
#unittests	57.88% <40.00%> (-0.10%)	⬇️

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

Impacted Files	Coverage Δ
mmdet/models/losses/iou_loss.py	29.94% <0.00%> (ø)
mmdet/datasets/pipelines/transforms.py	73.09% <66.66%> (-0.08%)	⬇️
mmdet/models/roi_heads/mask_scoring_roi_head.py	81.57% <0.00%> (-18.43%)	⬇️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	92.30% <0.00%> (-5.50%)	⬇️

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[hellock]

There are also other transforms like SegRescale which needs to be updated.

[hellock]

We need to update the minimum version of mmcv in requirements and readme.

[Johnson-Wang]

Since there is currently no version requirement for mmcv in requirements and readme, I can wait for the next version bump of mmdetection to merge this pr.