[pingpong-hip] main-mpi requires GPU-aware MPI

Although the general README mentions that certain benchmarks require GPU-aware MPI, it's not explicitly mentioned anywhere that pingpong-hip is one of those benchmarks. I have two suggestions:

1. Add a README.md file to pingpong-hip (and probably pingpong-cuda too) explicitly mentioning that it requires GPU-aware MPI.
2. Add a fallback implementation for main-mpi that doesn't require GPU-aware MPI. If it's available, we use GPU-aware MPI, if it's not, we explicitly do mem copies and use host pointers for MPI calls instead of device pointers. If this is of interest, I do have a patch for this that I can share.

Inspect the pingpong-hip benchmark, its proposed README.md, the likely pingpong-cuda README.md, and the main-mpi entry point. First determine how GPU-aware MPI is currently required, then clarify whether the work is documentation only or also the fallback path. Done means the requirement is explicit and, if implemented, main-mpi works with host-pointer copies when GPU-aware MPI is unavailable.