Accelerate video inference (#9487)

* The following code is slow, "self.cap.set(cv2.CAP_PROP_POS_FRAMES, self.vid_stride * (self.frame + 1)) # read at vid_stride".

* adjust...

* Update dataloaders.py

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/dataloaders.py

@@ -232,8 +232,9 @@ class LoadImages:
         if self.video_flag[self.count]:
             # Read video
             self.mode = 'video'
-            ret_val, im0 = self.cap.read()
-            self.cap.set(cv2.CAP_PROP_POS_FRAMES, self.vid_stride * (self.frame + 1))  # read at vid_stride
+            for _ in range(self.vid_stride):
+                self.cap.grab()
+            ret_val, im0 = self.cap.retrieve()
             while not ret_val:
                 self.count += 1
                 self.cap.release()