Add `GpuIqnHead::sync_target_from_online` (DtoD copy of online_params →
target_params) and call it from `FusedTrainingCtx::reset_for_fold`
alongside the existing IQN Adam reset.
Root cause: at fold boundary the main DQN does shrink-and-perturb +
hard target sync (gpu_dqn_trainer.rs:12495 — the latter explicitly added
to prevent fold-1 grad explosion). The IQN head was added later but
only had Polyak EMA `target_ema_update` — no hard-sync existed. Polyak
at τ=0.005/step closes ~0.5%/step, far too slow to close the
fold-boundary online↔target gap before inflated Bellman TD-error
compounds geometrically through the IQN backward pass.
Bug signature this resolves:
F0 ep5 Q=+0.79 (healthy)
F1 ep1 Q=+0.82 (boundary fine; c51_alpha warmup blends IQN low)
F1 ep2 Q=+2.23 (drift starts — c51_alpha ramp + IQN online↔target gap widens)
F1 ep3 Q=+4.05
F1 ep4 Q=+10.66 (geometric ~2.3×/epoch)
F1 ep5 NaN at step 5 (fp32 overflow past atom support)
Per `feedback_no_partial_refactor.md`: when a shared contract
(fold-boundary state migration) changes, every consumer must migrate
together. The main DQN sync was added explicitly — the IQN, TLOB, IQL,
and aux Adam states were added later but never extended to that
contract. This commit closes the IQN gap (primary). Aux Adam + IQN
readiness gaps follow in subsequent commits.
Touched:
- gpu_iqn_head.rs: new `sync_target_from_online` (mirrors gpu_dqn_trainer.rs:12495 exactly)
- fused_training.rs: call the new sync inside the existing IQN reset block
- docs/dqn-wire-up-audit.md: Invariant 7 entry covering Fix 1 of 3
cargo check -p ml --lib clean at 13 warnings (workspace baseline). No
fingerprint change. DtoD memcpy only — no HtoD/HtoH/DtoH paths.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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