Add stacking option to VectorSensorComponent (#5376)

com.unity.ml-agents/Editor/VectorSensorComponentEditor.cs

                 EditorGUILayout.PropertyField(so.FindProperty("m_SensorName"), true);
                 EditorGUILayout.PropertyField(so.FindProperty("m_ObservationSize"), true);
                 EditorGUILayout.PropertyField(so.FindProperty("m_ObservationType"), true);
+                EditorGUILayout.PropertyField(so.FindProperty("m_ObservationStacks"), true);
             }
             EditorGUI.EndDisabledGroup();
 

com.unity.ml-agents/Runtime/Sensors/VectorSensorComponent.cs

             set { m_ObservationType = value; }
         }
 
+        [HideInInspector, SerializeField]
+        [Range(1, 50)]
+        [Tooltip("Number of camera frames that will be stacked before being fed to the neural network.")]
+        int m_ObservationStacks = 1;
+
+        /// <summary>
+        /// Whether to stack previous observations. Using 1 means no previous observations.
+        /// Note that changing this after the sensor is created has no effect.
+        /// </summary>
+        public int ObservationStacks
+        {
+            get { return m_ObservationStacks; }
+            set { m_ObservationStacks = value; }
+        }
+
         /// <summary>
         /// Creates a VectorSensor.
         /// </summary>
             m_Sensor = new VectorSensor(m_ObservationSize, m_SensorName, m_ObservationType);
+            if (ObservationStacks != 1)
+            {
+                return new ISensor[] { new StackingSensor(m_Sensor, ObservationStacks) };
+            }
             return new ISensor[] { m_Sensor };
         }