[Perf] Skip Wan2.2 cross attn Ulysses SP (#2459)

Signed-off-by: gcanlin <canlinguosdu@gmail.com>

vllm_omni/diffusion/attention/layer.py

@@ -36,6 +36,7 @@ class Attention(nn.Module):
         scatter_idx: int = 2,
         gather_idx: int = 1,
         use_sync: bool = False,
+        skip_sequence_parallel: bool = False,
     ):
         super().__init__()
         self.attn_backend = get_attn_backend(-1)
@@ -62,6 +63,7 @@ class Attention(nn.Module):
         self.gather_idx = gather_idx
         self.use_sync = use_sync
         self.causal = causal
+        self.skip_sequence_parallel = skip_sequence_parallel
 
         self.use_ring = False
         self.ring_pg = None
@@ -98,6 +100,8 @@ class Attention(nn.Module):
         (e.g., in noise_refiner/context_refiner before unified_prepare in Z-Image).
         This avoids unnecessary SP communication for layers not covered by _sp_plan.
         """
+        if self.skip_sequence_parallel:
+            return self._no_parallel_strategy
         if is_forward_context_available():
             ctx = get_forward_context()
             if not ctx.sp_active:

vllm_omni/diffusion/models/wan2_2/wan2_2_transformer.py

@@ -539,6 +539,7 @@ class WanCrossAttention(nn.Module):
             num_kv_heads=self.num_heads,
             softmax_scale=1.0 / (head_dim**0.5),
             causal=False,
+            skip_sequence_parallel=True,
         )
 
     def forward(