[SYCL Spec][Joint Matrix] Add a new overload for joint_matrix_apply t…

…o be able to return result into a different matrix (#13153)

Currently, CUDA code that use this pattern:
for (int i = 0; i < c_frag.num_elements; i++) {
c_frag.x[i] = alpha * acc_frag.x[i] + beta * c_frag.x[i];
}
cannot be migrated to SYCL joint matrix.
This added overload addresses this limitation.

sycl/doc/extensions/experimental/sycl_ext_matrix/sycl_ext_oneapi_matrix.asciidoc

@@ -401,9 +401,14 @@ of the link:sycl_ext_intel_matrix.asciidoc[sycl_ext_intel_matrix]
 
 Besides the `Group` and the `joint_matrix` arguments,
 `joint_matrix_apply` takes a C++ Callable object which is invoked once
-for each element of the matrix. This callable object must be invocable
-with a single parameter of type `T&`. Commonly, applications pass a
-lambda expression.
+for each element of the matrix. There are two cases: (1) one matrix is
+passed, (2) two matrices are passed.
+
+===== Unary Operation
+In this case, `joint_matrix_apply` takes one `joint_matrix`
+argument. The callable object must be invocable with a single
+parameter of type `T&`. Commonly, applications pass a lambda
+expression.
 
 ```c++
 namespace sycl::ext::oneapi::experimental::matrix {
@@ -427,6 +432,39 @@ joint_matrix_apply(sg, C, [=](T &x) {
 });
 ```
 
+===== Binary Operation
+In this case, `joint_matrix_apply` takes two `joint_matrix` arguments:
+`jm0` and `jm1` that have the same `use`, number of rows, number of
+columns, and `layout`. `jm0` and `jm1` can be read-only, write-only,
+or read and write arguments. The callable object must be invocable
+with two parameters `x` and `y` of types `T0&` amd `T1&`, where `x` is
+an element from `jm0` and `y` is an element from `jm1`. Moreover, `x`
+and `y` are guaranteed to have identical coordinates in their
+respective matrices. Commonly, applications pass a lambda expression.
+
+```c++
+namespace sycl::ext::oneapi::experimental::matrix {
+
+template<typename Group, typename T0, typename T1, use Use,
+  size_t Rows, size_t Cols, layout Layout, typename F>
+void joint_matrix_apply(Group g,
+          joint_matrix<Group, T0, Use, Rows, Cols, Layout>& jm0,
+          joint_matrix<Group, T1, Use, Rows, Cols, Layout>& jm1,
+	  F&& func);
+
+} // namespace sycl::ext::oneapi::experimental::matrix
+```
+
+In the following example, every element `x` of the matrix `C` is
+multiplied by `alpha`. The result is returned into the element `y` of
+the matrix `D`.
+
+```c++
+joint_matrix_apply(sg, C, D, [=](const T &x, T &y) {
+    y = x * alpha;
+});
+```
+
 ==== Prefetch
 
 ```c++