Add section to save segmented frames as video in video_predictor_example.ipynb notebook

[siromermer]

Summary

This PR adds a new section to the video_predictor_example.ipynb notebook that saves segmented frames as a video file.

The notebook is designed for processing videos, but originally it only outputs a dictionary of masks without producing a video.
Adding a cell to save the frames in MP4 format makes it easier to visualize the segmentation results.

Testing

Tested with multiple videos locally — the generated MP4 files work as expected, with blue masks correctly overlayed.

[siromermer]

Hi @ronghanghu 👋
I’ve made some updates in this PR and would appreciate it if you could review them when you have time.
I thought it would be useful to add a video saving option after performing SAM2 segmentation on video input.
Thanks a lot!