Merge commit '4c060df2f17405dc26dc65a77e412d5c2a23525a'

Conflicts:
	subtree/rabit/tracker/rabit_yarn.py

subtree/rabit/src/allreduce_base.h

@@ -63,6 +63,10 @@ class AllreduceBase : public IEngine {
     if (world_size == -1) return 1;
     return world_size;
   }
+  /*! \brief whether is distributed or not */
+  virtual bool IsDistributed(void) const {
+    return tracker_uri == "NULL";
+  }
   /*! \brief get rank */
   virtual std::string GetHost(void) const {
     return host_uri;

subtree/rabit/src/engine_empty.cc

@@ -56,6 +56,10 @@ class EmptyEngine : public IEngine {
   virtual int GetWorldSize(void) const {
     return 1;
   }
+  /*! \brief whether it is distributed */
+  virtual bool IsDistributed(void) const {
+    return false;
+  }
   /*! \brief get the host name of current node */
   virtual std::string GetHost(void) const {
     return std::string("");

subtree/rabit/src/engine_mpi.cc

@@ -59,6 +59,10 @@ class MPIEngine : public IEngine {
   virtual int GetWorldSize(void) const {
     return MPI::COMM_WORLD.Get_size();
   }
+  /*! \brief whether it is distributed */
+  virtual bool IsDistributed(void) const {
+    return true;
+  }
   /*! \brief get the host name of current node */
   virtual std::string GetHost(void) const {
     int len;