[XPU] Update Bagel's flash_attn_varlen_func to fa utils (#1295)

Signed-off-by: zhenwei-intel <zhenwei.liu@intel.com>

vllm_omni/diffusion/attention/backends/utils/fa.py

@@ -30,6 +30,11 @@ if current_omni_platform.is_rocm():
             from aiter import flash_attn_func, flash_attn_varlen_func  # noqa: F401
     except (ImportError, ModuleNotFoundError):
         pass
+elif current_omni_platform.is_xpu():
+    try:
+        from vllm.v1.attention.backends.fa_utils import flash_attn_varlen_func  # noqa: F401
+    except (ImportError, ModuleNotFoundError):
+        pass
 else:
     # CUDA: try FA3 -> FA2 fallback chain
     # Try FA3 from fa3-fwd PyPI package