DENSE_DEVICE_SUM=4 assumed each rank's rows of x were the contiguous
[me*nrows,...) block; x and the slab columns are in global-site order
(hX[myGsite*nbasis+b]), the contiguous block is the rank-major index.
Coincide on one rank only -> Frontier VERIFY 0.9965. Stage via myGsite,
scatter through the inverse of BuildRankMajorMap. 4-rank laptop VERIFY
7.38e-07, identical to the allreduce path.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RAhdQHrkzKmzfxpxW1whLn
Peter Boyle
