fix: Initialize no_rope_layers when missing in Eagle3 config

Handles cases where draft model configs (like LlamaConfig) don't have
no_rope_layers attribute by initializing it with proper defaults before
Llama4DecoderLayer creation.

This prevents AttributeError during draft model initialization while
maintaining compatibility with existing configurations.

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: Rahul Tuli <rtuli@redhat.com>

Author: rahul-tuli

vllm/model_executor/models/llama4_eagle3.py

@@ -275,9 +275,16 @@ def _validate_and_update_config(
 
         # Draft model layer index is increased by start_layer_id,
         # so we need to pad relevant configs accordingly
-        self.config.no_rope_layers = [
-            0
-        ] * start_layer_id + self.config.no_rope_layers
+        if not hasattr(self.config, 'no_rope_layers'):
+            # Initialize no_rope_layers if it doesn't exist
+            # Default to all layers having rope (value of 1)
+            total_layers = start_layer_id + getattr(self.config, 'num_hidden_layers', 1)
+            self.config.no_rope_layers = [1] * total_layers
+        else:
+            # Pad existing no_rope_layers for the layer offset
+            self.config.no_rope_layers = [
+                0
+            ] * start_layer_id + self.config.no_rope_layers
 
         # Update quantization configuration for layer offset
         if isinstance(quant_config, TorchAOConfig):