Export c++ headers in CMake installation. (#4897)

* Move get transpose into cc.

* Clean up headers in host device vector, remove thrust dependency.

* Move span and host device vector into public.

* Install c++ headers.

* Short notes for c and c++.

Co-Authored-By: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

src/tree/constraints.cu

@@ -6,17 +6,16 @@
 #include <thrust/execution_policy.h>
 #include <thrust/iterator/counting_iterator.h>
 
-#include <xgboost/logging.h>
-
 #include <algorithm>
 #include <bitset>
 #include <string>
 #include <sstream>
 #include <set>
 
+#include "xgboost/logging.h"
+#include "xgboost/span.h"
 #include "constraints.cuh"
 #include "param.h"
-#include "../common/span.h"
 #include "../common/device_helpers.cuh"
 
 

src/tree/constraints.cuh

@@ -12,7 +12,7 @@
 #include <vector>
 
 #include "param.h"
-#include "../common/span.h"
+#include "xgboost/span.h"
 #include "../common/bitfield.h"
 #include "../common/device_helpers.cuh"
 

src/tree/split_evaluator.cc

@@ -3,10 +3,8 @@
  * \file split_evaluator.cc
  * \brief Contains implementations of different split evaluators.
  */
-#include "split_evaluator.h"
 #include <dmlc/json.h>
 #include <dmlc/registry.h>
-#include <xgboost/logging.h>
 #include <algorithm>
 #include <unordered_set>
 #include <vector>
@@ -15,9 +13,12 @@
 #include <string>
 #include <sstream>
 #include <utility>
+
+#include "xgboost/logging.h"
+#include "xgboost/host_device_vector.h"
 #include "param.h"
+#include "split_evaluator.h"
 #include "../common/common.h"
-#include "../common/host_device_vector.h"
 
 namespace dmlc {
 DMLC_REGISTRY_ENABLE(::xgboost::tree::SplitEvaluatorReg);

src/tree/tree_updater.cc

@@ -3,10 +3,10 @@
  * \file tree_updater.cc
  * \brief Registry of tree updaters.
  */
-#include <xgboost/tree_updater.h>
 #include <dmlc/registry.h>
 
-#include "../common/host_device_vector.h"
+#include "xgboost/tree_updater.h"
+#include "xgboost/host_device_vector.h"
 
 namespace dmlc {
 DMLC_REGISTRY_ENABLE(::xgboost::TreeUpdaterReg);

src/tree/updater_gpu_hist.cu

@@ -14,13 +14,15 @@
 #include <queue>
 #include <utility>
 #include <vector>
+
+#include "xgboost/host_device_vector.h"
+#include "xgboost/span.h"
+
 #include "../common/common.h"
 #include "../common/compressed_iterator.h"
 #include "../common/device_helpers.cuh"
 #include "../common/hist_util.h"
-#include "../common/host_device_vector.h"
 #include "../common/timer.h"
-#include "../common/span.h"
 #include "../data/ellpack_page.cuh"
 #include "param.h"
 #include "updater_gpu_common.cuh"