Bcast: scatter_allgather and scatter_allgather_ring expect N_elem >= N_procs
Allreduce: rabenseifner expects N_elem >= pow2 nearest to N_procs
In all cases, the implementations will fall back to a linear implementation,
which will most likely yield the worst performance (noted for 4B bcast on 128 ranks)
Signed-off-by: Joseph Schuchart <schuchart@icl.utk.edu>
(cherry picked from commit 04d198fc9f4b592fc512b27428d3158e279b51e1)
5e19de8366