[Fix] Unfinished label conversion from -1 to 255 in 1.x

[siddancha]

Motivation

This is motivated by a previously unfinished PR (#2332). In that PR, the label -1 was changed to 255 in BaseSegDataset, which is correct. However, it was changed at only one location. There is another location in mmseg/datasets/basesegdataset.py where -1 was still being used that was not converted to 255. I have now converted it to 255.

This is exactly same as a similar fix to the master branch via #2515 .

Modification

I've simply converted the snipped

if new_id != -1:
    new_palette.append(palette[old_id])

to

if new_id != 255:
    new_palette.append(palette[old_id])

Checklist

• Pre-commit or other linting tools are used to fix the potential lint issues.
  • I've fixed all linting/pre-commit errors.
• The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
  • _No unit tests need to be added or were affected.
• If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMDet3D.
  • I don't think this change affects MMDet or MMDet3D.
• The documentation has been modified accordingly, like docstring or example tutorials.
  • This change fixes an existing bug and doesn't require modifying any documentation/docstring.

[MeowZheng]

This modification looks good to me, and the ci failed probably because upstream repos mmdet or mmengine did some backward incompatible changes, and we are working on fix it.

[MeowZheng]

we have fixed ci problem in #2519 and please rebase the modification on it. :)

[siddancha]

Great! Merged the latest version of dev-1.x.

[codecov]

Codecov Report

Base: 83.39% // Head: 83.40% // Increases project coverage by +0.01% 🎉

Coverage data is based on head (e236cce) compared to base (b9b5d8b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #2516      +/-   ##
===========================================
+ Coverage    83.39%   83.40%   +0.01%     
===========================================
  Files          145      145              
  Lines         8510     8510              
  Branches      1274     1274              
===========================================
+ Hits          7097     7098       +1     
  Misses        1198     1198              
+ Partials       215      214       -1     

Flag	Coverage Δ
unittests	83.40% <100.00%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/datasets/basesegdataset.py	100.00% <100.00%> (+1.02%)	⬆️
mmseg/datasets/transforms/transforms.py	90.40% <0.00%> (ø)

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