add back ray import

Author: liljonnystyle

examples/train.py

@@ -255,6 +255,8 @@ def on_episode_end(info):
 
 def train_rllib(submodule, flags):
     """Train policies using the PPO algorithm in RLlib."""
+    import ray
+    
     flow_params = submodule.flow_params
     flow_params['sim'].render = flags.render
     policy_graphs = getattr(submodule, "POLICY_GRAPHS", None)