[mlir][mesh] Fix -Wunused-variable in Spmdization.cpp (NFC)

llvm-project/mlir/lib/Dialect/Mesh/Transforms/Spmdization.cpp:573:14:
 error: unused variable 'targetShardType' [-Werror,-Wunused-variable]
  ShapedType targetShardType =
             ^
1 error generated.

Author: DamonFool

mlir/lib/Dialect/Mesh/Transforms/Spmdization.cpp

@@ -570,7 +570,7 @@ reshardOn1DMesh(ImplicitLocOpBuilder &builder, ClusterOp mesh,
                 TypedValue<ShapedType> sourceShard) {
   assert(sourceShard.getType() ==
          shardShapedType(sourceUnshardedValue.getType(), mesh, sourceSharding));
-  ShapedType targetShardType =
+  [[maybe_unused]] ShapedType targetShardType =
       shardShapedType(sourceUnshardedValue.getType(), mesh, targetSharding);
   assert(sourceShard.getType().getRank() == targetShardType.getRank());
   assert(mesh.getRank() == 1 && "Only 1D meshes are currently supported.");