Use RAFT communicator instead of a separate communicator in WholeMemory

[teju85]

PR #3 introduces include/whole_memory.h which currently seems to have its own communicator. In the future, we should be using the raft communicator handle instead.

[teju85]

Similarly, there's also include/whole_nccl_memory.h which can be replaced with similar logic from RAFT too.

[BradReesWork]

still waiting on RAFT for host comms (using NCCL but with host copy)