Add YOLOv4 model

[dxoigmn]

Some options:

• https://github.com/Tianxiaomo/pytorch-YOLOv4
• https://github.com/AlexeyAB/Yet-Another-YOLOv4-Pytorch (forked from https://github.com/VCasecnikovs/Yet-Another-YOLOv4-Pytorch)

I'm partial to the AlexeyAB repo.

[dxoigmn]

I veto https://github.com/Tianxiaomo/pytorch-YOLOv4 because it isn't torch-like due its use of self.inference state that smells a lot like self.training. That will cause all kinds of headaches.

[dxoigmn]

Interestingly, https://github.com/AlexeyAB/Yet-Another-YOLOv4-Pytorch implements "Self adversial training with fgsm": AlexeyAB/Yet-Another-YOLOv4-Pytorch@ea3e0dc