sequence: fix bug when first partition vertices < 4096

[lorinlee]

1. When first partition's vertices size less than 4096, this alignment will cause poffset(1) = 0, and the next loop will start from v_i = 0 again. Finally, the partition offsets are [0, 0), [0, 0), [0, 0) ...
2. this alignment modifies offset, but not adjust amount
3. Why need this alignment ?

[yetingsky]

cc @xuguruogu