[BUG] Fix SPMM strided view (#2124)

With the bug fix #2117 there can be an issue with `z_tmp` memory being uninitialized.
SPMM formula is `Z = alpha . X * Y + beta . Z` so when `beta` is not zero, Z is being read.
The proposed solution in this PR remove the need for an extra allocation and a copy from/to an external buffer, by creating a strided view of the original Z.

Authors:
  - Micka (https://github.com/lowener)

Approvers:
  - Corey J. Nolet (https://github.com/cjnolet)

URL: #2124

cpp/include/raft/sparse/linalg/spmm.hpp

@@ -42,7 +42,7 @@ namespace linalg {
  * @param[in] x input raft::device_csr_matrix_view
  * @param[in] y input raft::device_matrix_view
  * @param[in] beta scalar
- * @param[out] z output raft::device_matrix_view
+ * @param[inout] z input-output raft::device_matrix_view
  */
 template <typename ValueType,
           typename IndexType,
@@ -60,19 +60,15 @@ void spmm(raft::resources const& handle,
 {
   bool is_row_major = detail::is_row_major(y, z);
 
-  auto z_tmp = raft::make_device_matrix<ValueType, IndexType>(handle, z.extent(0), z.extent(1));
   auto z_tmp_view = raft::make_device_strided_matrix_view<ValueType, IndexType, LayoutPolicyZ>(
-    z_tmp.data_handle(), z.extent(0), z.extent(1), is_row_major ? z.stride(0) : z.stride(1));
+    z.data_handle(), z.extent(0), z.extent(1), is_row_major ? z.stride(0) : z.stride(1));
 
   auto descr_x = detail::create_descriptor(x);
   auto descr_y = detail::create_descriptor(y);
   auto descr_z = detail::create_descriptor(z_tmp_view);
 
   detail::spmm(handle, trans_x, trans_y, is_row_major, alpha, descr_x, descr_y, beta, descr_z);
 
-  raft::copy(
-    z.data_handle(), z_tmp.data_handle(), z_tmp.size(), raft::resource::get_cuda_stream(handle));
-
   RAFT_CUSPARSE_TRY_NO_THROW(cusparseDestroySpMat(descr_x));
   RAFT_CUSPARSE_TRY_NO_THROW(cusparseDestroyDnMat(descr_y));
   RAFT_CUSPARSE_TRY_NO_THROW(cusparseDestroyDnMat(descr_z));