Fix hard-coded parameter name in gemma3n.py (#27946)

Signed-off-by: Seungduk Kim <seungduk.kim@yanolja.com>
Signed-off-by: Biswa Panda <biswa.panda@gmail.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Biswa Panda <biswa.panda@gmail.com>
Co-authored-by: Nicolò Lucchesi <nlucches@redhat.com>

vllm/model_executor/models/gemma3n.py

@@ -357,8 +357,27 @@ class Gemma3nAttention(nn.Module):
             offset = 2 if self.sliding_window is not None else 1
             kv_shared_layer_index = first_kv_shared_layer_idx - offset
             if kv_shared_layer_index >= 0:
+                # Different model wrappers expose layer parameters under
+                # different parent attributes.
+                # For example:
+                #   - Gemma3nForCausalLM → parameters live under "model.layers"
+                #   - Gemma3nForConditionalGeneration →
+                #     under "language_model.model.layers"
+                # This logic extracts the portion of the parameter name
+                # *before* ".layers."
+                # so downstream code can consistently reference the correct
+                # model root regardless of which wrapper class was used.
+                if ".layers." in prefix:
+                    param_name_before_layers = prefix.split(".layers.")[0]
+                else:
+                    raise ValueError(
+                        "Unexpected prefix format for Gemma3nAttention: "
+                        f"'{prefix}'. The prefix is expected to contain "
+                        "'.layers.' to correctly determine the KV sharing "
+                        "target layer."
+                    )
                 # Only the greater layer is required to specify sharing.
-                kv_sharing_target_layer_name = f"language_model.model.layers.{kv_shared_layer_index}.self_attn.attn"  # noqa: E501
+                kv_sharing_target_layer_name = f"{param_name_before_layers}.layers.{kv_shared_layer_index}.self_attn.attn"  # noqa: E501
 
         self.rotary_emb = get_rope(
             self.head_dim,