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/include/info.h

@@ -40,6 +40,11 @@ struct ncclInfo {
   // Algorithm details
   int chunkSteps;
   int sliceSteps;
+  // For alltoallv
+  const size_t *sendcounts;
+  const size_t *sdispls;
+  const size_t *recvcounts;
+  const size_t *rdispls;
   // Computed later
   int algorithm;
   int protocol;