Recommend a working conda installation sequence of torchvision, ffmpeg, opencv (both python and shared libraries + includes) together

Original context: https://github.com/pytorch/vision/issues/4240

Dfeault installation of torchvision and ffmpeg without version installs a non-workable ffmpeg 4.3 missing some libraries and gets installed for some reason from PyTorch channel.

I also read in other threads https://github.com/pytorch/vision/issues?q=is%3Aissue+is%3Aopen+ffmpeg that torchvision currently needs ffmpeg 4.2, but this is not reflected in dependency versions.

Also, building torchvision with ffmpeg from source is source of a lot of problems (in the issue search above)

Could README / torchvision:

1. recommend a known workable installation command of ffmpeg (maybe version-pinned? or with fixed dependency versions?)
2. recommend a known workable installation command of ffmpeg that lets you build torchvision from sources

At best it would be great to have a simple CI test installing from conda pytorch / torchvision / ffmpeg / opencv / maybe pillow-simd / open3d and a simple CI test for building from sources (as a regular user would).

@soumith