Gram matrix support for sparse input (rapidsai#1296)

This PR adds sparse input support (CSR) for GramMatrix kernel computation. This is a requirement to enable SVM support for sparse input in [cuML issue 2197](rapidsai/cuml#2197).

It also adds row norm computation for CSR which is utilized for expanded L2 norm computation within RBF kernels.

Although this branch introduces a new API it is still backwards compatible with the old GramMatrix API (which is marked as deprecated).

CC @cjnolet @tfeher

Authors:
  - Malte Förster (https://github.com/mfoerste4)
  - Corey J. Nolet (https://github.com/cjnolet)

Approvers:
  - Tamas Bela Feher (https://github.com/tfeher)
  - Corey J. Nolet (https://github.com/cjnolet)

URL: rapidsai#1296

cpp/include/raft/core/device_mdspan.hpp

@@ -255,6 +255,36 @@ auto make_device_matrix_view(ElementType* ptr, IndexType n_rows, IndexType n_col
   return device_matrix_view<ElementType, IndexType, LayoutPolicy>{ptr, extents};
 }
 
+/**
+ * @brief Create a 2-dim mdspan instance for device pointer with a strided layout
+ *        that is restricted to stride 1 in the trailing dimension. It's
+ *        expected that the given layout policy match the layout of the underlying
+ *        pointer.
+ * @tparam ElementType the data type of the matrix elements
+ * @tparam IndexType the index type of the extents
+ * @tparam LayoutPolicy policy for strides and layout ordering
+ * @param[in] ptr on device to wrap
+ * @param[in] n_rows number of rows in pointer
+ * @param[in] n_cols number of columns in pointer
+ * @param[in] stride leading dimension / stride of data
+ */
+template <typename ElementType, typename IndexType, typename LayoutPolicy = layout_c_contiguous>
+auto make_device_strided_matrix_view(ElementType* ptr,
+                                     IndexType n_rows,
+                                     IndexType n_cols,
+                                     IndexType stride)
+{
+  constexpr auto is_row_major = std::is_same_v<LayoutPolicy, layout_c_contiguous>;
+  IndexType stride0           = is_row_major ? (stride > 0 ? stride : n_cols) : 1;
+  IndexType stride1           = is_row_major ? 1 : (stride > 0 ? stride : n_rows);
+
+  assert(is_row_major ? stride0 >= n_cols : stride1 >= n_rows);
+  matrix_extent<IndexType> extents{n_rows, n_cols};
+
+  auto layout = make_strided_layout(extents, std::array<IndexType, 2>{stride0, stride1});
+  return device_matrix_view<ElementType, IndexType, layout_stride>{ptr, layout};
+}
+
 /**
  * @brief Create a 1-dim mdspan instance for device pointer.
  * @tparam ElementType the data type of the vector elements