Add inference step after training

[robmarkcole]

As its always nice to see. In my case performed with:

!python /content/deepstack-trainer/detect.py --weights /content/deepstack-trainer/train-runs/exp/weights/best.pt --img 640 --conf 0.25 --source /content/valid/images/

This batch processes all the images in /content/valid/images/. Then the images are in a directory /content/runs/detect/exp