Three fixes:
1. FusedTrainingCtx::new now accepts RegimeConditionalDQN by using
primary_head() — the old code rejected it, causing silent fallback
to non-fused train_step() which has action-space mismatch with
branching DQN (5-action Q-table vs 45-action factored indices →
CUDA_ERROR_ILLEGAL_ADDRESS buffer overrun)
2. ensure_fused_ctx() returns Result instead of () — fused init
failure is now a hard error (no silent CPU fallback)
3. GpuTensor::cat now supports dim>0 concatenation (was unimplemented,
caused "dim=1 > 0 not yet implemented" error in validation path)
Root cause chain:
RegimeConditional rejected by fused init
→ silent fallback to DQN::train_step()
→ compute_loss_internal() uses num_actions=5 but actions are 0-44
→ gather with out-of-bounds offsets → CUDA_ERROR_ILLEGAL_ADDRESS
→ async error poisons CUDA context
→ next stream.synchronize() deadlocks forever
Remaining: curiosity kernel crash also poisons the stream. The
curiosity training runs inside collect_gpu_experiences() and its
async error blocks the PER insert. This needs separate investigation
of curiosity_training_kernel.cu.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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