Add CPU send/recv for pipeline (#5315)
* cpu send/recv
* clean up send/recv
* remove unused code
* assert and nccl option for mnist
* add build option to enable build with only cpu. Without this, nccl is always enabled which will break build on machine that only contains cpu
* Add USE_MPI distinct from USE_NCCL/USE_HOROVOD
* fix
* fix
* exclude cpu send/recv for machines without mpi
Co-authored-by: Tim Harris <tiharr@microsoft.com>
