As far as I know, object detection isn’t officially built into fastai2 yet.
I think it’s on the roadmap (there are some hints that it’s coming, eg the get_annotations function exists) but I haven’t seen anything newer in the official docs or repo though.
Looks like someone is working on it though; this looks like this notebook has a partially complete RetinaNet implementation in fastai2 (referenced via this thread): https://github.com/muellerzr/Practical-Deep-Learning-for-Coders-2.0/blob/master/Computer%20Vision/06_Object_Detection.ipynb