[diffusion] chore: ensure CFG Zero Star numerical stability for Helios model (#20091)

Signed-off-by: Lancer <maruixiang6688@gmail.com>

python/sglang/multimodal_gen/runtime/pipelines_core/stages/model_specific_stages/helios_denoising.py

@@ -29,6 +29,8 @@ logger = init_logger(__name__)
 
 def optimized_scale(positive_flat, negative_flat):
     """CFG Zero Star: compute optimal guidance scale."""
+    positive_flat = positive_flat.float()
+    negative_flat = negative_flat.float()
     dot_product = torch.sum(positive_flat * negative_flat, dim=1, keepdim=True)
     squared_norm = torch.sum(negative_flat**2, dim=1, keepdim=True) + 1e-8
     return dot_product / squared_norm