fix sequence parallel(Ulysses) grad scale for zero0

[inkcherry]

use dp_world_size for grad reduction, instead of seq_dp_world_size. Currently, for zero0, only sparse tensors use the correct world_size.

tiny model with sp=4 grad norm test:

grad_norm	step1	step2	step3	step4	step5	step100
zero1	15.825	16.646	15.853	16.159	17.333	15.555
zero0	3.956	4.161	3.963	4.040	4.333	3.889
zero0(this patch)	15.825	16.646	15.853	16.159	17.333	15.554