ColorMode.Segmentation for Videos/Webcam not supported yet as per detectron2 documentation

## 🚀 Feature
Hi @ppwwyyxx and other contributors !!
i have gone through the documentation... where ColorMode.SEGMENTATION part is shown and also in the " Source code for **detectron2.utils.video_visualizer class** " it is written that there is no support for ColorMode.SEGMENTATION for videos yet... (only ColorMode.SEGMENTATION works on images) ... Can you please tell me any alternative way or some other techniques to get same class/instances colors on Videos/webcam also in detectron2.

## Motivation & Examples
This feature will be very much useful in case of Videos/Webcam input. till now this feature of ColorMode.SEGMENTATION is only available for IMAGES and NOT for Videos/Webcam inputs

## Tell us why the feature is useful.

if this feature will be implemented then we will be able to view same colors for all the classes in VIDEOS/WEBCAM.
in case of instance segmentation.

Start with the detectron2.utils.video_visualizer class source and the documentation's ColorMode.SEGMENTATION behavior for images, then trace the video and webcam rendering path. Define the color-consistency behavior for classes or instances and verify that videos and webcam input produce the same colors as the image mode.

Written by the indexing model from the issue text.