update 3dball agent take discrete action

com.unity.ml-agents/Runtime/Training/ReplayBuffer.cs

             var samples = new List<Transition>(batchSize);
             for (var i = 0; i < batchSize; i++)
             {
-                samples.Add(m_Buffer[m_CurrentIndex-1]);
+                samples.Add(m_Buffer[0]);
             }
             return samples;
         }

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

             }
 
             var samples = m_Buffer.SampleBatch(m_Config.batchSize);
-            // m_ModelRunner.UpdateModel(samples);
+            m_ModelRunner.UpdateModel(samples);
 
             // Update target network
             // if (m_TrainingStep % m_Config.updateTargetFreq == 0)