Add multi-stream saving feature (#3864)

* Added the recording feature for multiple streams

Thanks for the very cool repo!!
I was trying to record multiple feeds at the same time, but the current version of the detector only had one video writer and one vid_path!
So the streams were not being saved and only were initialized with one frame and this process didn't record the whole thing.

Fix:
I made a list of `vid_writer` and `vid_path` and the `i` from the loop over the `pred` took care of the writer which need to work!

I hope this helps, Thanks!

* Cleanup list lengths

* batch size variable

* Update datasets.py

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

utils/datasets.py

@@ -352,7 +352,7 @@ class LoadStreams:  # multiple IP or RTSP cameras
         return self.sources, img, img0, None
 
     def __len__(self):
-        return 0  # 1E12 frames = 32 streams at 30 FPS for 30 years
+        return len(self.sources)  # 1E12 frames = 32 streams at 30 FPS for 30 years
 
 
 def img2label_paths(img_paths):