Fix: Align forward and backward arguments in cpp_extension tutorial

Author: ZhigaoyuanWang

advanced_source/cpp_extension.rst

@@ -1147,7 +1147,7 @@ on it:
     auto d_old_h = d_X.slice(/*dim=*/1, 0, state_size);
     auto d_input = d_X.slice(/*dim=*/1, state_size);
 
-    return {d_old_h, d_input, d_weights, d_bias, d_old_cell, d_gates};
+    return {d_old_h, d_input, d_weights, d_bias, d_old_cell};
   }