[Sparse] Add cpu compilation support (#6310)

dgl_sparse/CMakeLists.txt

@@ -38,6 +38,7 @@ set(SPARSE_INCLUDE "${CMAKE_CURRENT_SOURCE_DIR}/include")
 file(GLOB SPARSE_HEADERS ${SPARSE_INCLUDE})
 file(GLOB SPARSE_SRC
   ${SPARSE_DIR}/*.cc
+  ${SPARSE_DIR}/cpu/*.cc
 )
 add_library(${LIB_DGL_SPARSE_NAME} SHARED ${SPARSE_SRC} ${SPARSE_HEADERS})
 target_include_directories(

dgl_sparse/src/cpu/matrix_ops_impl.cc

+/**
+ *  Copyright (c) 2023 by Contributors
+ * @file cpu/matrix_ops_impl.cc
+ * @brief DGL C++ matrix operators.
+ */
+#include "./matrix_ops_impl.h"
+
+namespace dgl {
+namespace sparse {}  // namespace sparse
+}  // namespace dgl

dgl_sparse/src/matrix_ops_impl.h

+/**
+ *  Copyright (c) 2023 by Contributors
+ * @file matrix_ops_impl.h
+ * @brief DGL C++ sparse matrix operator implementations.
+ */
+#ifndef DGL_SPARSE_MATRIX_OPS_IMPL_H_
+#define DGL_SPARSE_MATRIX_OPS_IMPL_H_
+
+namespace dgl {
+namespace sparse {}
+}  // namespace dgl
+
+#endif  // DGL_SPARSE_MATRIX_OPS_IMPL_H_