Fix uneven head sequence parallelism bug (#6774)

[Eugene29]

Here gather_idx < 2 represents is_first_all2all. During the first all2all, uneven_head_all2all will be called if either num_heads % seq_world_size != 0 or get_num_kv_heads() is None.

During the second all2all, it'll return return uneven_head_all2all if and only if get_num_kv_heads() is None which is always set during the first uneven all2all. This means that there will no longer be issue where uneven_head_all2all is returned for the second all2all because of num_heads % seq_world_size != 0.

Fixes: #6774

[Eugene29]

Fix #6774

[inkcherry]

LGTM， Many thanks!