bce_loss_multi_horizon: fused forward+backward, block tree-reduce (no
atomicAdd), NaN labels masked (drop). Loss = mean over valid; grad =
(p-y)/(p(1-p)) scaled by 1/N_valid.
adamw_step: element-wise AdamW with weight decay; one thread per param.
Tests pass on sm_86:
BCE (4/4): positive+finite loss, near-zero loss when probs match
labels, analytic grad matches GPU-computed finite-difference at
eps=1e-3 / max_relative=5e-2 across 5 perturbation points, NaN
labels mask grad and contribute zero to loss/N_valid.
AdamW (4/4): zero-grad moves param only by weight-decay, positive
grad decreases param, step counter increments, repeated descent
on grad=theta drives ‖θ‖ from 40 to <1 in 200 steps.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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