[Fix] Relax the requirement of PALETTE in dataset wrappers

[gaotongxiao]

Motivation

The recent new feature #6716 introduced a new assumption on the PALETTE attribute in all dataset wrappers, which breaks external references to these wrappers without such an attribute in their datasets. This PR turns this requirement optional.

Modification

Dataset wrappers take PALETTE from their dataset classes only if available, or None will be used to fill out the attribute otherwise.

BC-breaking (Optional)

None

[codecov]

Codecov Report

Merging #7085 (b00e9fe) into dev (4bdb312) will decrease coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev    #7085      +/-   ##
==========================================
- Coverage   62.41%   62.13%   -0.29%     
==========================================
  Files         330      330              
  Lines       26199    26018     -181     
  Branches     4436     4436              
==========================================
- Hits        16353    16165     -188     
- Misses       8976     8979       +3     
- Partials      870      874       +4     

Flag	Coverage Δ
unittests	62.11% <100.00%> (-0.29%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/datasets/dataset_wrappers.py	82.22% <100.00%> (-0.30%)	⬇️
mmdet/utils/misc.py	95.23% <0.00%> (-4.77%)	⬇️
mmdet/core/post_processing/bbox_nms.py	74.62% <0.00%> (-4.48%)	⬇️
mmdet/models/dense_heads/base_mask_head.py	40.00% <0.00%> (-4.45%)	⬇️
mmdet/models/seg_heads/base_semantic_head.py	34.37% <0.00%> (-3.87%)	⬇️
mmdet/models/roi_heads/base_roi_head.py	76.47% <0.00%> (-3.53%)	⬇️
mmdet/core/bbox/coder/yolo_bbox_coder.py	50.00% <0.00%> (-2.95%)	⬇️
mmdet/models/detectors/base.py	50.78% <0.00%> (-2.56%)	⬇️
...panoptic_fusion_heads/base_panoptic_fusion_head.py	52.63% <0.00%> (-2.37%)	⬇️
mmdet/datasets/pipelines/formatting.py	65.78% <0.00%> (-1.98%)	⬇️
... and 62 more

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