Add CUDA iterator to tensor view. (#10074)

include/xgboost/linalg.h

@@ -295,6 +295,9 @@ class TensorView {
   using ShapeT = std::size_t[kDim];
   using StrideT = ShapeT;
 
+  using element_type = T;                  // NOLINT
+  using value_type = std::remove_cv_t<T>;  // NOLINT
+
  private:
   StrideT stride_{1};
   ShapeT shape_{0};
@@ -314,7 +317,7 @@ class TensorView {
   }
 
   template <size_t old_dim, size_t new_dim, int32_t D, typename I>
-  LINALG_HD size_t MakeSliceDim(size_t new_shape[D], size_t new_stride[D],
+  LINALG_HD size_t MakeSliceDim(std::size_t new_shape[D], std::size_t new_stride[D],
                                 detail::RangeTag<I> &&range) const {
     static_assert(new_dim < D);
     static_assert(old_dim < kDim);
@@ -528,9 +531,10 @@ class TensorView {
   LINALG_HD auto Stride(size_t i) const { return stride_[i]; }
 
   /**
-   * \brief Number of items in the tensor.
+   * @brief Number of items in the tensor.
    */
   [[nodiscard]] LINALG_HD std::size_t Size() const { return size_; }
+  [[nodiscard]] bool Empty() const { return Size() == 0; }
   /**
    * \brief Whether this is a contiguous array, both C and F contiguous returns true.
    */
@@ -865,7 +869,9 @@ class Tensor {
   auto HostView() { return this->View(DeviceOrd::CPU()); }
   auto HostView() const { return this->View(DeviceOrd::CPU()); }
 
-  [[nodiscard]] size_t Size() const { return data_.Size(); }
+  [[nodiscard]] std::size_t Size() const { return data_.Size(); }
+  [[nodiscard]] bool Empty() const { return Size() == 0; }
+
   auto Shape() const { return common::Span<size_t const, kDim>{shape_}; }
   auto Shape(size_t i) const { return shape_[i]; }
 

include/xgboost/span.h

@@ -701,10 +701,10 @@ class IterSpan {
     return {data() + _offset, _count == dynamic_extent ? size() - _offset : _count};
   }
   [[nodiscard]] XGBOOST_DEVICE constexpr iterator begin() const noexcept {  // NOLINT
-    return {this, 0};
+    return it_;
   }
   [[nodiscard]] XGBOOST_DEVICE constexpr iterator end() const noexcept {  // NOLINT
-    return {this, size()};
+    return it_ + size();
   }
 };
 }  // namespace common