Fix img_norm in hrnet when using fcos detector

[v-qjqs]

This PR fix #4232, fix different img_norm in hrnet when using fcos.

[codecov]

Codecov Report

Merging #4250 (783f04c) into master (c551b59) will decrease coverage by 0.18%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4250      +/-   ##
==========================================
- Coverage   62.70%   62.51%   -0.19%     
==========================================
  Files         235      232       -3     
  Lines       17821    17597     -224     
  Branches     2984     2942      -42     
==========================================
- Hits        11174    11001     -173     
+ Misses       6103     6060      -43     
+ Partials      544      536       -8     

Flag	Coverage Δ
unittests	62.48% <ø> (-0.18%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/mask_scoring_roi_head.py	55.35% <0.00%> (-32.15%)	⬇️
mmdet/models/roi_heads/test_mixins.py	45.73% <0.00%> (-8.53%)	⬇️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	94.62% <0.00%> (-5.38%)	⬇️
mmdet/datasets/samplers/distributed_sampler.py	29.41% <0.00%> (-3.93%)	⬇️
mmdet/models/roi_heads/mask_heads/fcn_mask_head.py	70.55% <0.00%> (-1.23%)	⬇️
mmdet/models/roi_heads/dynamic_roi_head.py	25.00% <0.00%> (-0.38%)	⬇️
mmdet/models/necks/fpn.py	93.82% <0.00%> (-0.23%)	⬇️
mmdet/models/backbones/resnet.py	99.30% <0.00%> (ø)
mmdet/models/backbones/__init__.py	100.00% <0.00%> (ø)
mmdet/models/detectors/__init__.py	100.00% <0.00%> (ø)
... and 9 more

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[hellock]

There are linting errors.