Document Eagle3 auxiliary layer default selection in Llama

Add documentation explaining that get_eagle3_aux_hidden_state_layers()
provides default layer selection and that the GPU model runner can
override this with values from speculative config for dynamic
configuration.

Author: rahul-tuli

vllm/model_executor/models/llama.py

@@ -578,6 +578,11 @@ def set_aux_hidden_state_layers(self, layers: tuple[int, ...]) -> None:
         self.model.aux_hidden_state_layers = layers
 
     def get_eagle3_aux_hidden_state_layers(self) -> tuple[int, ...]:
+        """Override to return default layers for Llama
+
+        Note: The GPU model runner will override this with layers from
+        the speculative config if available, providing dynamic configuration.
+        """
         num_layers = len(self.model.layers)
         return (2, num_layers // 2, num_layers - 3)