fix gemma3 results all zero (#17364)

Signed-off-by: mayuyuace <qiming1.zhang@intel.com>

vllm/model_executor/layers/layernorm.py

@@ -241,6 +241,9 @@ class GemmaRMSNorm(CustomOp):
         """PyTorch-native implementation equivalent to forward()."""
         orig_dtype = x.dtype
         if residual is not None:
+            if orig_dtype == torch.float16:
+                x = x + residual.float()
+            else:
                 x = x + residual
             residual = x