Add detect.py GIF video inference (#6410)

* Add detect.py GIF video inference

* Cleanup

detect.py

@@ -199,7 +199,7 @@ def run(weights=ROOT / 'yolov5s.pt',  # model.pt path(s)
                             h = int(vid_cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
                         else:  # stream
                             fps, w, h = 30, im0.shape[1], im0.shape[0]
-                            save_path += '.mp4'
+                        save_path = str(Path(save_path).with_suffix('.mp4'))  # force *.mp4 suffix on results videos
                         vid_writer[i] = cv2.VideoWriter(save_path, cv2.VideoWriter_fourcc(*'mp4v'), fps, (w, h))
                     vid_writer[i].write(im0)