[Fix] Fix PALETTE AttributeError in single_gpu_test

[shinya7y]

This PR aims to fix #7224 and open-mmlab/mmdetection3d#1259

#7085 has fixed the same errors in dataset_wrappers.py, but the error in single_gpu_test remains.

[AronLin]

Thanks for your contribution!

[codecov]

Codecov Report

Merging #7230 (027d2a4) into master (37a1150) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #7230      +/-   ##
==========================================
+ Coverage   62.15%   62.18%   +0.02%     
==========================================
  Files         330      330              
  Lines       26033    26033              
  Branches     4437     4437              
==========================================
+ Hits        16182    16188       +6     
+ Misses       8978     8975       -3     
+ Partials      873      870       -3     

Flag	Coverage Δ
unittests	62.16% <0.00%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
mmdet/apis/test.py	13.39% <0.00%> (ø)
mmdet/core/bbox/assigners/max_iou_assigner.py	73.68% <0.00%> (+1.31%)	⬆️
mmdet/models/dense_heads/base_dense_head.py	90.34% <0.00%> (+1.70%)	⬆️
mmdet/core/bbox/samplers/random_sampler.py	80.55% <0.00%> (+5.55%)	⬆️

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