`use_iqn` is exactly the `use_/enable_` boolean banned by
`feedback_no_feature_flags.md`. It gated dead code: production training
runs IQN unconditionally through `cuda_pipeline/gpu_iqn_head.rs` +
`iqn_dual_head_kernel`, properly wired into the branching architecture
with the `FIXED_TAUS` 5-quantile schedule. Nothing in the cuda_pipeline
production path ever read `use_iqn` or `iqn_network`.
The legacy `iqn_network: Option<QuantileNetwork>` field on `DQN` was a
parallel CPU-side network from a pre-branching era, structurally
unreachable in production: every consumer was gated behind the
`if true /* use_branching: always on */` arm at `q_values_for_batch`,
so the IQN else-if at L1822 was dead code. Training optimised
`iqn_network` parameters in isolation; inference never read them. That's
the train/inference mismatch the L283 comment ("IQN trains base
q_network but inference uses dist_dueling network (zero gradients)")
was working around by **disabling** the feature instead of fixing the
inference path. Per `feedback_no_quickfixes.md` + `feedback_no_hiding.md`
the fix is to remove the dead path entirely.
Strip:
- `DQNConfig::use_iqn` field + parses + checkpoint hash + metadata.
`dqn.use_iqn` / `dqn.iqn_embedding_dim` / `dqn.iqn_num_quantiles` /
`dqn.iqn_kappa` from older checkpoints are silently dropped on load
(same pattern used for `use_dueling`). `iqn_lambda` stays — the
cuda_pipeline dual head consumes it as the IQN aux loss weight.
- 3 vestigial config fields (`iqn_num_quantiles`, `iqn_kappa`,
`iqn_embedding_dim`) — never read in production; kernel-side macros
(`IQN_NUM_QUANTILES = 5`, embed_dim 64, kappa 1.0) are the actual
config.
- 4 default builders (`Default`, `aggressive`, `conservative`,
`emergency_safe_defaults`) drop the 4 IQN-related fields each.
- `DQN::iqn_network` field + initialisation block in `new_with_stream`.
- 6 conditional gates in `select_action`, `select_action_with_confidence`,
`select_action_inference`, `q_values_for_batch` — all collapse to the
live (branching or standard-Q) arm.
- `DQN::get_state_embedding` (only consumed by deleted IQN paths).
- The entire `crates/ml-dqn/src/quantile_regression.rs` module (392 LOC)
+ its 2 lib.rs exports. Nothing outside `ml-dqn` ever imported it
(the `quantile_huber_loss` reference in `gpu_iqn_head.rs` is a CUDA
kernel name string, unrelated to this Rust module).
Downstream call sites:
- `crates/ml/src/trainers/dqn/{config,fused_training,trainer/constructor}.rs`:
drop `iqn_num_quantiles` / `iqn_embedding_dim` / `iqn_kappa` references
off `DQNConfig`; substitute kernel-fixed literals (64, 1.0) where
`GpuDqnTrainConfig` / `GpuIqnConfig` still expect them.
- `crates/ml/examples/evaluate_baseline.rs`: drop two `iqn_num_quantiles`
hyperparam reads (their `..DQNConfig::default()` fallbacks now stand
alone).
- `crates/ml/tests/dqn_action_collapse_fix_test.rs`: drop the
`assert!(!config.use_iqn, "...gradient dead zone")` and the explicit
`config.use_iqn = false` setter; the dead-zone pathology is now
structurally impossible.
- `crates/ml/tests/dqn_inference_test.rs`: drop `config.use_iqn = false`.
- `services/trading_service/src/services/dqn_model.rs`: drop the
`iqn={}` debug-log field.
- `crates/ml/src/trainers/dqn/distributional_q_tests.rs`: ship Test 0.F
(Plan A Task 8 #186) — converged-checkpoint extraction harness +
`MappedF32Buffer` (mapped pinned f32 mirror) + `compute_sigma_c51_test`
kernel handle. The structural assertions panic on the local 5-epoch
smoke checkpoint as designed (under-converged: sigma_C51 spread ~1.4%
across directions, P(active)=0.4330 >= 0.20). Docstring rewritten to
drop `use_iqn=false` framing and the legacy "Tier-B-prime" caveat;
Tier-A version is GPU-integration-only per
`feedback_no_cpu_forwards.md` (CPU is read-only).
`docs/dqn-wire-up-audit.md` updated per Invariant 7.
Build: `cargo check --workspace --tests` clean (0 errors).
Test: `cargo test -p ml --lib distributional_q_tests::test_0f
-- --ignored --nocapture` produces bit-identical sigma_C51 /
argmax / Thompson values vs pre-removal — confirms branching
forward path is the same code post-cleanup as pre-cleanup
(legacy `use_iqn` arms were unreachable, as expected).
Net: 12 files, +458 / -785 lines (327 LOC deleted).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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