further cleanup of single process multi-GPU code (#4810)

* use subspan in gpu predictor instead of copying
* Revise `HostDeviceVector`

tests/cpp/common/test_device_helpers.cu

@@ -10,17 +10,6 @@
 
 using xgboost::common::Span;
 
-struct Shard { int id; };
-
-TEST(DeviceHelpers, Basic) {
-  std::vector<Shard> shards (4);
-  for (int i = 0; i < 4; ++i) {
-    shards[i].id = i;
-  }
-  int sum = dh::ReduceShards<int>(&shards, [](Shard& s) { return s.id ; });
-  ASSERT_EQ(sum, 6);
-}
-
 void CreateTestData(xgboost::bst_uint num_rows, int max_row_size,
                     thrust::host_vector<int> *row_ptr,
                     thrust::host_vector<xgboost::bst_uint> *rows) {