[SYCL][Graph] Fix post-commit CI after PR #11472 (#12362)

Ignore unused `Size` parameter from
`memory_manager::ext_oneapi_fill_cmd_buffer`.

This was introduced in PR #11472 and
passed as `Req->MMemoryRange` from the command-group requirement but not
used for calling the underlying UR function, leading to a
`-Wunused-parameter` error.

It may be that this is needed to support buffers of dimension greater
than 1, which there are currently no graphs tests for. This will be
investigated as a follow-up once post-commit CI is unblocked with this
PR.

Author: EwanC

sycl/source/detail/memory_manager.cpp

@@ -1692,6 +1692,7 @@ void MemoryManager::ext_oneapi_fill_cmd_buffer(
     unsigned int ElementSize,
     std::vector<sycl::detail::pi::PiExtSyncPoint> Deps,
     sycl::detail::pi::PiExtSyncPoint *OutSyncPoint) {
+  (void)Size;
   assert(SYCLMemObj && "The SYCLMemObj is nullptr");
 
   const PluginPtr &Plugin = Context->getPlugin();