Minor code refactor for sorted_order (#8326)

the fast-path for single column sort will not use any sorted_indices, this PR just move the sorted_indices behind the single column sort code block

Authors:
  - Bobby Wang (https://github.com/wbo4958)

Approvers:
  - Mark Harris (https://github.com/harrism)
  - Nghia Truong (https://github.com/ttnghia)

URL: #8326

cpp/src/sort/sort_impl.cuh

@@ -107,14 +107,6 @@ std::unique_ptr<column> sorted_order(table_view input,
                  "Mismatch between number of columns and null_precedence size.");
   }
 
-  std::unique_ptr<column> sorted_indices = cudf::make_numeric_column(
-    data_type(type_to_id<size_type>()), input.num_rows(), mask_state::UNALLOCATED, stream, mr);
-  mutable_column_view mutable_indices_view = sorted_indices->mutable_view();
-  thrust::sequence(rmm::exec_policy(stream),
-                   mutable_indices_view.begin<size_type>(),
-                   mutable_indices_view.end<size_type>(),
-                   0);
-
   // fast-path for single column sort
   if (input.num_columns() == 1 and not cudf::is_nested(input.column(0).type())) {
     auto const single_col = input.column(0);
@@ -124,6 +116,14 @@ std::unique_ptr<column> sorted_order(table_view input,
                   : sorted_order<false>(single_col, col_order, null_prec, stream, mr);
   }
 
+  std::unique_ptr<column> sorted_indices = cudf::make_numeric_column(
+    data_type(type_to_id<size_type>()), input.num_rows(), mask_state::UNALLOCATED, stream, mr);
+  mutable_column_view mutable_indices_view = sorted_indices->mutable_view();
+  thrust::sequence(rmm::exec_policy(stream),
+                   mutable_indices_view.begin<size_type>(),
+                   mutable_indices_view.end<size_type>(),
+                   0);
+
   auto flattened = structs::detail::flatten_nested_columns(input, column_order, null_precedence);
   auto& input_flattened     = std::get<0>(flattened);
   auto device_table         = table_device_view::create(input_flattened, stream);