Upgrading to NCCL2 (#3404)

* Upgrading to NCCL2

* Part - II of NCCL2 upgradation

 - Doc updates to build with nccl2
 - Dockerfile.gpu update for a correct CI build with nccl2
 - Updated FindNccl package to have env-var NCCL_ROOT to take precedence

* Upgrading to v9.2 for CI workflow, since it has the nccl2 binaries available

* Added NCCL2 license + copy the nccl binaries into /usr location for the FindNccl module to find

* Set LD_LIBRARY_PATH variable to pick nccl2 binary at runtime

* Need the nccl2 library download instructions inside Dockerfile.release as well

* Use NCCL2 as a static library

src/tree/updater_gpu_hist.cu

@@ -810,6 +810,7 @@ class GPUHistMaker : public TreeUpdater {
   }
 
   void AllReduceHist(int nidx) {
+    reducer_.GroupStart();
     for (auto& shard : shards_) {
       auto d_node_hist = shard->hist.GetHistPtr(nidx);
       reducer_.AllReduceSum(
@@ -818,6 +819,7 @@ class GPUHistMaker : public TreeUpdater {
           reinterpret_cast<GradientPairSumT::ValueT*>(d_node_hist),
           n_bins_ * (sizeof(GradientPairSumT) / sizeof(GradientPairSumT::ValueT)));
     }
+    reducer_.GroupEnd();
 
     reducer_.Synchronize();
   }