Fork 3 CUDA streams at CublasForward construction. In forward_online_raw
and forward_online_f32, the trunk records an event, each branch stream
waits on it, submits its GEMM+bias ops on its own stream (via
cublasSetStream), then the main stream joins all three. This overlaps
the 3 independent advantage head computations (exposure, order, urgency)
that previously executed sequentially.
Safety: a distinct_branches guard checks h_b0!=h_b1!=h_b2 at runtime;
callers that alias branch hidden buffers (Pass 3 Double DQN scratch
reuse) fall back to the sequential loop. CUDA Graph capture (CUDA 12+)
captures the fork/join pattern as graph dependencies.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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