[Epilogue] Fix VisitorRowReduction helper return type

[XFDG]

Summary

• make VisitorRowReduction::Callbacks::reduction return void, matching its side-effect-only implementation and sole call site
• remove undefined behavior from falling off the end of a non-void device function
• preserve the existing element-wise register reduction logic unchanged

Testing

• B200 (SM100a), CUDA 13.1: an explicit instantiation of the helper produces NVCC diagnostic #940-D: missing return statement at end of non-void function on current main
• B200 (SM100a), CUDA 13.1: the same probe builds and runs with exit code 0 after the fix, with no target return diagnostic

Contribution disclosure

AI assistance was used for repository research, implementation, and running the checks above.

Fixes #3495