Hi Tom,
I do not think there is a way to use pad_packed_seq if you want to concat two tensor “vertically”. In my example, the upper line (abcdefg000) could not be combined with the lower line (u for any point, 0 for any padding). So what I have to do is to manually pad the upper sequence (abcdefg => abcdefg000) and find someway to do stack with u.
The second solution seems to be working. I will give it a shot today and let you know.
Thanks!