[Fix] Enable roi_head test empty rois

[OceanPang]

Enable roi_head test empty rois.

[codecov]

Codecov Report

Merging #5081 (f9fe078) into master (2a856ef) will decrease coverage by 0.61%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5081      +/-   ##
==========================================
- Coverage   65.50%   64.89%   -0.62%     
==========================================
  Files         265      267       +2     
  Lines       20315    20642     +327     
  Branches     3419     3476      +57     
==========================================
+ Hits        13308    13395      +87     
- Misses       6250     6492     +242     
+ Partials      757      755       -2     

Flag	Coverage Δ
unittests	64.86% <100.00%> (-0.62%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/test_mixins.py	52.32% <100.00%> (ø)
mmdet/models/utils/transformer.py	44.91% <0.00%> (-41.46%)	⬇️
mmdet/models/necks/channel_mapper.py	77.41% <0.00%> (-22.59%)	⬇️
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	70.74% <0.00%> (-1.87%)	⬇️
mmdet/models/detectors/__init__.py	100.00% <0.00%> (ø)
mmdet/models/dense_heads/__init__.py	100.00% <0.00%> (ø)
mmdet/models/dense_heads/deformable_detr_head.py	33.60% <0.00%> (ø)
mmdet/models/detectors/deformable_detr.py	100.00% <0.00%> (ø)
mmdet/models/dense_heads/detr_head.py	89.44% <0.00%> (+0.66%)	⬆️

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