Fix AttributeError and add colab link

[hhaAndroid]

resolve: #4603

[codecov]

Codecov Report

Merging #4643 (4f0110c) into master (df681d8) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4643      +/-   ##
==========================================
- Coverage   64.12%   64.06%   -0.06%     
==========================================
  Files         251      251              
  Lines       19347    19347              
  Branches     3265     3265              
==========================================
- Hits        12406    12395      -11     
- Misses       6289     6302      +13     
+ Partials      652      650       -2     

Flag	Coverage Δ
unittests	64.03% <ø> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
mmdet/models/roi_heads/test_mixins.py	45.03% <0.00%> (-8.40%)	⬇️

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