Add `out_of_sample_predictions` accessor function?

[ablaom]

To support external control of iterative models that can more efficiently compute predictions on an evaluation set internally. See the discussion at https://discourse.julialang.org/t/ann-learnapi-jl-proposal-for-a-basement-level-machine-learning-api/93048/12?u=ablaom

[ablaom]

Added in 0.2. This is just called LearnAPI.predictions