Adding YOLOFastestV1 full model except loss

[hglee98]

Description

First of all, Thanks to @aychun for supporting the implementation. Most of it are from @aychun 's endeavor.
This PR aims to support YOLOFastest model (#376).
However, There are some works left to do.

• Implement correspoding loss (for now I choose to use existing retinanetloss).
• Load pretrained weights
• Add docs

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Change(s)

• Add DarkNet, YOLOv3FPN, yolo_fastest_head
• Add corresponding config file

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