Make HostDeviceVector single gpu only (#4773)

* Make HostDeviceVector single gpu only

src/gbm/gbtree.cc

@@ -182,9 +182,9 @@ void GBTree::DoBoost(DMatrix* p_fmat,
     CHECK_EQ(in_gpair->Size() % ngroup, 0U)
         << "must have exactly ngroup*nrow gpairs";
     // TODO(canonizer): perform this on GPU if HostDeviceVector has device set.
-    HostDeviceVector<GradientPair> tmp
-        (in_gpair->Size() / ngroup, GradientPair(),
-         GPUDistribution::Block(in_gpair->Distribution().Devices()));
+    HostDeviceVector<GradientPair> tmp(in_gpair->Size() / ngroup,
+                                       GradientPair(),
+                                       in_gpair->DeviceIdx());
     const auto& gpair_h = in_gpair->ConstHostVector();
     auto nsize = static_cast<bst_omp_uint>(tmp.Size());
     for (int gid = 0; gid < ngroup; ++gid) {