Add Owlv2; Add option to use owlvit / owlv2's image preprocessing procedure (the roi_align in current implementation processes images differently and results in distribution shifts and subpar performance)

[xuanlinli17]

• Factor in a bug found by Fix bug with non square images #20
• Add Owlv2
• Add an option (no_roi_align) in the OwlPredictor class of nanoowl/owl_predictor.py to use the original owlvit / owlv2 implementation's image preprocessing procedure. From my debugging, I find that the current nanoowl repo preprocesses images differently from the original owlvit / owlv2 implementations due to the use of "roi_align". This causes the larger models (base-16, large-14) to perform subpar compared to the original owlvit / owlv2. Passing in no_roi_align=True fixes this issue.
• Add an option (nms_threshold) to use non-maximal suppression bounding box filtering
• Fix cv2 "image no write permission" error in drawing utils