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

src/misc/argcheck.cc

@@ -54,7 +54,11 @@ ncclResult_t ArgsCheck(struct ncclInfo* info) {
   if (info->coll == ncclCollAllGather || info->coll == ncclCollReduceScatter
     || info->coll == ncclCollGather || info->coll == ncclCollScatter || info->coll == ncclCollAllToAll)
     info->nBytes *= info->comm->nRanks; // count is per rank
-
+  if (info->coll == ncclCollAllToAllv) {
+    // Use count to store data type size for alltoallv
+    info->count = ncclTypeSize(info->datatype);
+    info->datatype = ncclInt8;
+  }
   if (info->op < 0 || info->op >= ncclNumOps) {
     WARN("%s : invalid reduction operation %d", info->opName, info->op);
     return ncclInvalidArgument;