[models] Implement latest detection SOTA models

[frgfm]

Being among the new SOTA models, the library should include:

• YOLOv3 from https://arxiv.org/abs/1804.02767
• Scaled YOLOv4 from https://arxiv.org/abs/2011.08036
• YOLOX from https://arxiv.org/abs/2107.08430