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/compressed_iterator.h

@@ -63,7 +63,7 @@ class CompressedBufferWriter {
    * \fn  static size_t CompressedBufferWriter::CalculateBufferSize(int
    * num_elements, int num_symbols)
    *
-   * \brief Calculates number of bytes requiredm for a given number of elements
+   * \brief Calculates number of bytes required for a given number of elements
    * and a symbol range.
    *
    * \author  Rory
@@ -74,7 +74,6 @@ class CompressedBufferWriter {
    *
    * \return  The calculated buffer size.
    */
-
   static size_t CalculateBufferSize(size_t num_elements, size_t num_symbols) {
     const int bits_per_byte = 8;
     size_t compressed_size = static_cast<size_t>(std::ceil(
@@ -188,7 +187,7 @@ class CompressedIterator {
 
  public:
   CompressedIterator() : buffer_(nullptr), symbol_bits_(0), offset_(0) {}
-  CompressedIterator(CompressedByteT *buffer, int num_symbols)
+  CompressedIterator(CompressedByteT *buffer, size_t num_symbols)
       : buffer_(buffer), offset_(0) {
     symbol_bits_ = detail::SymbolBits(num_symbols);
   }

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: