empty video output for Kinectics demo

Hi

First of all, thanks a lot for making this available.

I am trying to run a demo on a random video with the kinetics dataset using the SLOWFAST_8x8_R50.pkl checkpoint and running the demo like this:

`python3.8 tools/run_net.py --cfg demo/Kinetics/SLOWFAST_8x8_R50.yaml DEMO.INPUT_VIDEO /tmp/jVkw5f1uxTw_2.mp4 DEMO.OUTPUT_FILE /tmp/a.mp4`
`
My input video is a 32 frame video from my own dataset.

The demo runs OK, without errors, but the result (/tmp/a.mp4) is an empty mp4 (it is recognized as an mp4 file but it is empty). Not sure if I should format the input video differently or what else I could be doing wrong.

Thanks a lot