Gradient based sampling for GPU Hist (#5093)

* Implement gradient based sampling for GPU Hist tree method.
* Add samplers and handle compacted page in GPU Hist.

src/common/device_helpers.cuh

@@ -1266,6 +1266,26 @@ thrust::device_ptr<T const> tcend(xgboost::HostDeviceVector<T> const& vector) {
   return tcbegin(vector) + vector.Size();
 }
 
+template <typename T>
+thrust::device_ptr<T> tbegin(xgboost::common::Span<T>& span) {  // NOLINT
+  return thrust::device_ptr<T>(span.data());
+}
+
+template <typename T>
+thrust::device_ptr<T> tend(xgboost::common::Span<T>& span) {  // // NOLINT
+  return tbegin(span) + span.size();
+}
+
+template <typename T>
+thrust::device_ptr<T const> tcbegin(xgboost::common::Span<T> const& span) {
+  return thrust::device_ptr<T const>(span.data());
+}
+
+template <typename T>
+thrust::device_ptr<T const> tcend(xgboost::common::Span<T> const& span) {
+  return tcbegin(span) + span.size();
+}
+
 template <typename FunctionT>
 class LauncherItr {
 public: