[xla-auto-sharding] Fix potential dangling pointer (reference) bug.

Note:
- The scaled request object is allocated on the stack when `ScaleRequest` is called, but then it goes out of scope.
- This change should be equally performant due to return value optimization.
PiperOrigin-RevId: 705277876

xla/hlo/experimental/auto_sharding/auto_sharding_solver.cc

@@ -539,7 +539,7 @@ void AddMemoryTerms(
 absl::StatusOr<AutoShardingSolverOutput> FormulateAndSolveMIPFromSolverRequest(
     const AutoShardingSolverRequest& unscaled_request) {
   const absl::Time start_time = absl::Now();
-  const AutoShardingSolverRequest& request = ScaleRequest(unscaled_request);
+  const AutoShardingSolverRequest request = ScaleRequest(unscaled_request);
   const size_t num_edges = request.edges_size();
   const int num_workers = 32;
   // SAT or SCIP