Update urls to download.openmmlab.com

[xvjiarui]

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[codecov]

Codecov Report

Merging #3665 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3665      +/-   ##
==========================================
- Coverage   61.31%   61.28%   -0.03%     
==========================================
  Files         213      213              
  Lines       14531    14531              
  Branches     2441     2441              
==========================================
- Hits         8910     8906       -4     
- Misses       5177     5181       +4     
  Partials      444      444              

Flag	Coverage Δ
#unittests	61.28% <ø> (-0.03%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/detectors/cornernet.py	94.87% <0.00%> (-5.13%)	⬇️
mmdet/models/dense_heads/corner_head.py	74.31% <0.00%> (-1.92%)	⬇️
mmdet/models/roi_heads/test_mixins.py	53.76% <0.00%> (+5.37%)	⬆️

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