Add coco object detection dataset

[simonschoelly]

What has changed and why?

• Added COCO format input for object detection
• Added some tests for get_class_id_to_internal_class_id_mapping because that method was not covered anymore after this PR.
• Had to disable the test_file_locking_concurrent_increments test on Python 3.8 because it kept failing even after restarting the tests.

How has it been tested?

• Wrote some tests to cover the new dataset
• Wrote a test that runs the train loop with a COCO dataset.

Did you update CHANGELOG.md?

• Yes
• Not needed (internal change)

Did you update the documentation?

Coming in a follow up PR

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[simonschoelly]

/review

[Copilot]

Pull request overview

This PR adds COCO-format dataset support for object detection and refactors YOLO object detection to reuse a new shared ObjectDetectionDataset implementation, with accompanying test updates and a changelog entry.

Changes:

• Add COCOObjectDetectionDataArgs/COCOObjectDetectionDatasetArgs and support selecting COCO vs YOLO via a discriminated union in the train task config.
• Introduce a generic ObjectDetectionDataset and switch YOLO object detection dataset args to use it.
• Add/adjust tests for COCO datasets, label helper mapping, and object detection training (YOLO + COCO); skip a flaky file-locking test on Python 3.8.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
tests/helpers.py	Adds a helper to generate minimal COCO-format object detection datasets for tests.
tests/_data/test_yolo_object_detection_dataset.py	Updates YOLO tests to validate list_image_info() output format.
tests/_data/test_object_detection_dataset.py	Adds coverage for the new shared ObjectDetectionDataset behavior.
tests/_data/test_label_helpers.py	Adds tests for get_class_id_to_internal_class_id_mapping.
tests/_data/test_coco_object_detection_dataset.py	Adds tests for COCO dataset args list_image_info() normalization and class remapping.
tests/_commands/test_train_task.py	Splits YOLO OD training test and adds an end-to-end COCO OD training test.
tests/_commands/test_common_helpers.py	Skips a flaky multiprocessing file-locking test on Python 3.8.
src/lightly_train/_data/yolo_object_detection_dataset.py	Removes the dedicated YOLO dataset class and routes YOLO args to the shared dataset.
src/lightly_train/_data/object_detection_dataset.py	Introduces shared dataset implementation for object detection samples and transforms.
src/lightly_train/_data/coco_object_detection_dataset.py	Adds COCO dataset args + parsing logic for COCO JSON annotations.
src/lightly_train/_commands/train_task.py	Enables discriminated-union parsing for object detection data config (YOLO vs COCO).
CHANGELOG.md	Documents COCO object detection dataset format support.

[Copilot]

Pull request overview

Copilot reviewed 12 out of 12 changed files in this pull request and generated 4 comments.