* 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>