Allow multiple target proxy support with `predict` dispatching on the proxy.

[ablaom]

https://discourse.julialang.org/t/ann-learnapi-jl-proposal-for-a-basement-level-machine-learning-api/93048/12?u=ablaom

@CameronBieganek @juliohm

[ablaom]

In this way we can also get rid of the separate predict_joint - and even transform and inverse_transform, by adding special TargetProxy types for them?

[CameronBieganek]

Hmm, I'm not sure about getting rid of transform.

[ablaom]

closed as done