add target network prototype

com.unity.ml-agents/Runtime/Trainer.cs

         public int bufferSize = 1024;
         public int batchSize = 64;
         public float gamma = 0.99f;
+        public int updateTargetFreq = 200;
     }
 
     internal class Trainer: IDisposable
+        TrainingModelRunner m_TargetModelRunner;
         string m_behaviorName;
         TrainerConfig m_Config;
         int m_TrainingStep;
             m_behaviorName = behaviorName;
             m_Buffer = new ReplayBuffer(m_Config.bufferSize);
             m_ModelRunner = new TrainingModelRunner(actionSpec, model, seed);
+            m_TargetModelRunner = new TrainingModelRunner(actionSpec, model, seed);
+            // copy weights from model to target model
+            // m_TargetModelRunner.model.weights = m_ModelRunner.model.weights
             Academy.Instance.TrainerUpdate += Update;
         }
 
 
             // loss = MSE(q_values, expected_q_values);
             // m_ModelRunner.model = Barracuda.ModelUpdate(m_ModelRunner.model, loss);
+
+
+            // Update target network
+            if (m_TrainingStep % m_Config.updateTargetFreq == 0)
+            {
+                // copy weights
+            }
 
             m_TrainingStep += 1;
         }