YOLOV8 port to keras-hub

[oarriaga]

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This PR ports YOLOV8 from keras-cv to keras-hub (https://github.com/keras-team/keras-hub/issues/176). All necessary YOLOV8 functions are now found inside keras-hub:

• Add CIOU loss.
• Add missing masking functionality in the bounding_boxes module.
• Add multibackend non maximum supression layer.
• Add label encoder.
• Build basic abstract object detector task class.
• Add YOLOV8 backbone and detector.

Missing steps include:

• Upload previous presets to Kaggle.
• Remove skipping tests with presets.
• Add colab with basic functionality.
• Add weight transfer script from keras-cv to keras-hub.
• Add training script.