implementation of run_step_fast

[zhyx12]

The code in run_step_fast(), doesn't match the formula in annotation. This part aims at updating a_us_t[idx_c_a,:]

below is the annotation
update A_t(i->idx_c_a) = r_a * A_t(i->idx_c_a) + r_b * A_t(i->idx_c_b) (fast algorithm)
but code doesn't not contain r_a and r_b, which looks like:
A_us_t:indexAdd(1, torch.LongTensor{idx_c_a}, A_us_t:index(1, torch.LongTensor{idx_c_b}))

this makes me confused, should I change the code to annotation version(implementation in function merge_two_clusters())?

Thanks for your work!