Don't build video decoder homemade backend by default

[NicolasHug]

This PR avoids building the C++ video decoder by default and remove the installation of ffmpeg when we build a wheel.
As a result, the "homemade C++" backend of torchvision is removed from official releases. Some of the video decoding interface is still available, but only through an optional dependency on pyav. Eventually we'll want to remove everything, but this PR is an intermediate step that:

• removes the FFmpeg dependency, which was a major pain point
• doesn't touch anything on fbcode (which is important, because changes to fbcode are tricky to address).

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/vision/9208

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

This comment was automatically generated by Dr. CI and updates every 15 minutes.