Add set_cuda_backend Context Manager to publicly expose the BETA CUDA Interface

[NicolasHug]

Meant to be used as:

with torchcodec.set_cuda_backend("beta"):
    decoder = VideoDecoder("video.mp4", device="cuda")

decoder.get_frame_at(...)

[Dan-Flores]

Do we plan on changing the default to be the beta interface at some point in the future? Would we need to indicate the change to users in some way?

[NicolasHug]

Yes, we'll want the current "beta" to eventually become the default interface. Usually the way to give notice is that for a few versions, we'll be raising a warning if the user doesn't explicitly requests either the "beta" or the "ffmpeg" interface. Basically we'd force them to use the CM, or they get a warning. Then we do the switch and remove the warning.

This is a bit disruptive and annoying, maybe we'll decide that the new interface is just strictly better and 99.9999% compatible with the existing one, at which point we could decide not to raise any warning and just silently do the switch.