Previously the fold loop swallowed every training error into f64::NAN,
unconditionally pushed a value at the top of the loop, and then asserted
only !fold_losses.is_empty() — a tautology that could never fail. Any
CUDA error, NaN explosion, or regression of the zero-copy path passed
silently.
Now:
- Error propagation via `?` (no swallow). A zero-copy test can't
tolerate training failures — if training blew up, the zero-copy
wiring is broken and must surface.
- All fold losses must be finite and non-negative.
- Every fold must report epochs_trained >= 1 (zero-epoch fold =
kernel skipped or buffer not populated).
A direct "no htod/dtoh copy happened" assertion would need a copy-counter
instrumented into the fused-training GPU path plus a field on
TrainingMetrics. That is out of scope for this test; documented inline.
Verified PASS on laptop (RTX 3050 Ti):
loss=0.0018366, epochs=1
2570fe0130