Add Alltoallv RCCL kernel implementation (#269)

* Add alltoallv API and implementation

* Extend Rome P2P channel limit to multinode and alltoall kernels

* topo_expl: fix compilation and sync up with main

* gtest: use RCCL alltoallv API

* Code review changes

[ROCm/rccl commit: b871ea3c0c]

projects/rccl/src/graph/connect.cc

@@ -291,7 +291,7 @@ ncclResult_t ncclTopoPostset(struct ncclComm* comm, int* firstRanks, struct nccl
 
   int nc = nChannels*2;
   if (gcn == 908) nc = std::max(nc, 4);
-  if (comm->topo->nodes[NET].count == 0 && comm->topo->type == RCCL_TOPO_CR8G) nc = nChannels*3;
+  if (comm->topo->nodes[NET].count == 0 && comm->topo->type == RCCL_TOPO_CR8G) nc = nChannels*4;
   if (comm->topo->nodes[NET].count && comm->topo->type == RCCL_TOPO_4P2H_ROME) nc = 4*comm->topo->nodes[NET].count;
   int end = std::min((int)ncclMaxNchannels(), std::max(nc, ncclMinNchannels()));