Move DDQN forward (Pass 3) from double_dqn_stream to main stream.
NVIDIA docs: "bit-wise reproducibility is valid only when a single
CUDA stream is active." The double_dqn_stream fork is kept but
no longer used for cuBLAS operations.
Combined with AlgoGetIds (verified: same algo_id=16 every run),
this satisfies all documented NVIDIA preconditions for bit-wise
reproducibility. Epochs 1-3 are identical across runs. Epoch 4+
divergence remains under investigation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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