fix: correct reference to optimizer in update_optimizer_gen method

Author: bnb32

sup3r/models/abstract.py

@@ -355,7 +355,7 @@ def update_optimizer_gen(self, **kwargs):
         conf = self._optimizer_config.copy()
         conf.update(**kwargs)
         self._optimizer_config = conf
-        if self.optimizer is not None:
+        if self._optimizer is not None:
             self._optimizer = self.optimizer.__class__.from_config(conf)
 
     @property