Completes the IQN integration:
- rl_iqn_backward.cu: backprop through quantile embedding + output
projection to produce per-batch weight gradients for all 4 weight
tensors. Block per batch, HIDDEN_DIM threads.
- rl_iqn_loss.cu (already existed): quantile Huber loss feeds
grad_online_q into the backward kernel.
- rl_ensemble_action_value.cu: E_ensemble = α×C51 + (1-α)×IQN,
ISV-driven α at slot 544.
- Adam updates for IQN weights after backward.
- Ensemble Q feeds rl_q_pi_agree_b diagnostic.
Full stack smoke: 200 steps, l_q=2.43, no crash. Both C51 and IQN
learning simultaneously from the same replay transitions.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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