tailor and probably are built around models with a single outcome
column and a single .pred/.pred_class prediction column. A kerasnip
multi-output model (e.g. a recipe with output_1 + output_2 ~ .) instead
produces .pred_output_1, .pred_output_2, ... columns from multiple
truth columns in one fit — the standard parsnip::maybe_multivariate()
shape, but one tailor::fit()/workflows::add_tailor() call cannot
consume it (it selects outcome/estimate via [[, which requires
exactly one column).
kerasnip_output_view() wraps a fitted multi-output workflow together
with one output name, presenting it as if it were an ordinary
single-output fit: predict() returns standard .pred / .pred_class /
.pred_<level> columns for that output alone, letting you calibrate or
post-process each output separately with the usual tailor/probably
calls (see vignette("multi_output_postprocessing")).
