[IR][RmsNorm] pass None if not has_weight (#38961)

Signed-off-by: Linkun Chen <github@lkchen.net>
Signed-off-by: Luka Govedič <ProExpertProg@users.noreply.github.com>
Co-authored-by: Luka Govedič <ProExpertProg@users.noreply.github.com>

vllm/model_executor/layers/layernorm.py

@@ -241,8 +241,12 @@ class RMSNorm(CustomOp):
     ) -> torch.Tensor | tuple[torch.Tensor, torch.Tensor]:
         """PyTorch-native implementation equivalent to forward()."""
         if residual is None:
+            # TODO(luka): address the weight=None passing issue more generally
             return ir.ops.rms_norm(
-                x, self.weight.data, self.variance_epsilon, self.variance_size_override
+                x,
+                self.weight.data if self.has_weight else None,
+                self.variance_epsilon,
+                self.variance_size_override,
             )
 
         return self.forward_static(