[BUG] Cityscapes importer not reading train_extra images

[charitarthchugh]

Describe the problem

The train_extra split is not parsed by fiftyone. Looking at fiftyone/utils/cityscapes.py there seems to be no handling for this split.

Code to reproduce issue

fiftyone zoo datasets load cityscapes -k source_dir=.

With PWD having: gtCoarse.zip gtFine_trainvaltest.zip leftImg8bit_trainextra.zip leftImg8bit_trainvaltest.zip

System information

• OS Platform and Distribution (e.g., Linux Ubuntu 22.04): Arch Linux Rolling
• Python version (python --version): 3.11-3.13
• FiftyOne version (fiftyone --version): 1.4
• FiftyOne installed from (pip or source): pip

Willingness to contribute

The FiftyOne Community encourages bug fix contributions. Would you or another
member of your organization be willing to contribute a fix for this bug to the
FiftyOne codebase?

• Yes. I can contribute a fix for this bug independently
• Yes. I would be willing to contribute a fix for this bug with guidance
  from the FiftyOne community
• No. I cannot contribute a bug fix at this time

I am already working on a fix, will post PR soon.

[charitarthchugh]

waiting on #5630 to make PR.