The object detection will be covered in part 2 : Object detection in fast.ai v1
While waiting you can watch the part 2 of last year fast.ai MOOC lesson 8 and 9 to understand how it works. It used SSD for object detection.
If you need something works first. I recommend this repo: https://github.com/qfgaohao/pytorch-ssd . It is in pure pytorch
Hope that helps,