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/common/device_helpers.cuh

@@ -946,6 +946,24 @@ class AllReducer {
 #endif
   }
 
+  /**
+   * \brief Use in exactly the same way as ncclGroupStart
+   */
+  void GroupStart() {
+#ifdef XGBOOST_USE_NCCL
+    dh::safe_nccl(ncclGroupStart());
+#endif
+  }
+
+  /**
+   * \brief Use in exactly the same way as ncclGroupEnd
+   */
+  void GroupEnd() {
+#ifdef XGBOOST_USE_NCCL
+    dh::safe_nccl(ncclGroupEnd());
+#endif
+  }
+
   /**
    * \brief Allreduce. Use in exactly the same way as NCCL but without needing
    * streams or comms.