No longer in use

This is error prone. Suppose that now we want to support the nested types, it will be difficult to trace back to this line of code to change it. So I would rather implement the code path ready for nested types here, like what it was before:

if (has_nested_columns) {
    auto const key_equal = row_comp.equal_to<true>(has_nulls, null_equality::EQUAL, value_comp);
    map.insert(pair_iter, pair_iter + input.num_rows(), key_hasher, key_equal, stream.value());
  } else {
    auto const key_equal = row_comp.equal_to<false>(has_nulls, null_equality::EQUAL, value_comp);
    map.insert(pair_iter, pair_iter + input.num_rows(), key_hasher, key_equal, stream.value());
  }

Good point about the implicit use of non-nested key equality. Instead of adding a code path for nested types that isn't used by any existing code, I suggest leaving the code as is and adding a CUDF_EXPECTS check to ensure no nested types are involved in the calculation. This approach also avoids templating functions, which can help reduce build time.