CRITICAL BUG: When use_branching=true, the optimizer trains ONLY the
branching network, but q_values_for_batch() (used by walk-forward
evaluator) was reading from dist_dueling_q_network — which was never
trained. Walk-forward was evaluating random/untrained weights.
Fix: q_values_for_batch() now uses the exposure branch Q-values
[batch, 5] from the trained branching network when use_branching=true.
Also adds device/dtype migration to match forward() contract.
Fixed IQN test that implicitly had use_branching=true (default) with
num_actions=3 — incompatible with 5-action branching exposure head.
1331 tests pass (416 ml-dqn + 915 ml), 0 failures.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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