I haven’t tested yet but I guess this post can help you. https://forums.fast.ai/t/how-to-get-an-empty-convlearner-for-single-image-prediction/28245/50 . This is for single image but I think I will work with a batch of image too. Load the model you have trained and predict your images in pure Pytorch .
I am facing a similar problem and so do many people. I guess fastai will make a friendly API for this soon 