use generation_node_name on the GeneratorRun (facebook#2003)

Summary:

Replace the use of generation_step_index with generation_node_name because GenerationStrategy should no longer access index

Differential Revision: D51432075

ax/modelbridge/generation_strategy.py

@@ -510,7 +510,7 @@ def _get_model_state_from_last_generator_run(self) -> Dict[str, Any]:
         model_on_curr = self._curr.model_enum
         if (
             lgr is not None
-            and lgr._generation_step_index == self._curr.index
+            and lgr._generation_node_name == self._curr.node_name
             and lgr._model_state_after_gen
         ):
             if self.model or isinstance(model_on_curr, ModelRegistryBase):