bring back root reference

Project/Assets/ML-Agents/Examples/PushBlock/Prefabs/PushBlockCollabAreaGrid.prefab

   m_Modification:
     m_TransformParent: {fileID: 8188317207052398481}
     m_Modifications:
+    - target: {fileID: 1548337883655231979, guid: ac01d0f42c5e1463e943632a60d99967,
+        type: 3}
+      propertyPath: m_RootReference
+      value: 
+      objectReference: {fileID: 8190299122290044757}
     - target: {fileID: 2598450485826216109, guid: ac01d0f42c5e1463e943632a60d99967,
         type: 3}
       propertyPath: m_Model
     type: 3}
   m_PrefabInstance: {fileID: 6067781793364901444}
   m_PrefabAsset: {fileID: 0}
-  m_GameObject: {fileID: 0}
+  m_GameObject: {fileID: 8190299122290044757}
   m_Enabled: 1
   m_EditorHideFlags: 0
   m_Script: {fileID: 11500000, guid: d94a85eca2e074578943301959c555ba, type: 3}
     type: 3}
   m_PrefabInstance: {fileID: 6067781793364901444}
   m_PrefabAsset: {fileID: 0}
+--- !u!1 &8190299122290044757 stripped
+GameObject:
+  m_CorrespondingSourceObject: {fileID: 2710286047221272849, guid: ac01d0f42c5e1463e943632a60d99967,
+    type: 3}
+  m_PrefabInstance: {fileID: 6067781793364901444}
+  m_PrefabAsset: {fileID: 0}
 --- !u!1001 &6565363751102736699
 PrefabInstance:
   m_ObjectHideFlags: 0
     m_Modifications:
+    - target: {fileID: 1548337883655231979, guid: ac01d0f42c5e1463e943632a60d99967,
+        type: 3}
+      propertyPath: m_RootReference
+      value: 
+      objectReference: {fileID: 9115291448867436586}
     - target: {fileID: 2598450485826216109, guid: ac01d0f42c5e1463e943632a60d99967,
         type: 3}
       propertyPath: m_Model
     type: 3}
   m_PrefabInstance: {fileID: 6565363751102736699}
   m_PrefabAsset: {fileID: 0}
-  m_GameObject: {fileID: 0}
+  m_GameObject: {fileID: 9115291448867436586}
   m_Enabled: 1
   m_EditorHideFlags: 0
   m_Script: {fileID: 11500000, guid: d94a85eca2e074578943301959c555ba, type: 3}
     type: 3}
   m_PrefabInstance: {fileID: 6565363751102736699}
   m_PrefabAsset: {fileID: 0}
+--- !u!1 &9115291448867436586 stripped
+GameObject:
+  m_CorrespondingSourceObject: {fileID: 2710286047221272849, guid: ac01d0f42c5e1463e943632a60d99967,
+    type: 3}
+  m_PrefabInstance: {fileID: 6565363751102736699}
+  m_PrefabAsset: {fileID: 0}
 --- !u!1001 &6716844123244810954
 PrefabInstance:
   m_ObjectHideFlags: 0
     m_Modifications:
+    - target: {fileID: 1548337883655231979, guid: ac01d0f42c5e1463e943632a60d99967,
+        type: 3}
+      propertyPath: m_RootReference
+      value: 
+      objectReference: {fileID: 8695281997955662811}
     - target: {fileID: 2598450485826216109, guid: ac01d0f42c5e1463e943632a60d99967,
         type: 3}
       propertyPath: m_Model
     type: 3}
   m_PrefabInstance: {fileID: 6716844123244810954}
   m_PrefabAsset: {fileID: 0}
-  m_GameObject: {fileID: 0}
+  m_GameObject: {fileID: 8695281997955662811}
   m_Enabled: 1
   m_EditorHideFlags: 0
   m_Script: {fileID: 11500000, guid: d94a85eca2e074578943301959c555ba, type: 3}
     type: 3}
   m_PrefabInstance: {fileID: 6716844123244810954}
   m_PrefabAsset: {fileID: 0}
+--- !u!1 &8695281997955662811 stripped
+GameObject:
+  m_CorrespondingSourceObject: {fileID: 2710286047221272849, guid: ac01d0f42c5e1463e943632a60d99967,
+    type: 3}
+  m_PrefabInstance: {fileID: 6716844123244810954}
+  m_PrefabAsset: {fileID: 0}

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

                 var gridSize2d = new Vector3Int(gridSize.vector3IntValue.x, 1, gridSize.vector3IntValue.z);
                 var newGridSize = EditorGUILayout.Vector3IntField("Grid Size", gridSize2d);
                 gridSize.vector3IntValue = new Vector3Int(newGridSize.x, 1, newGridSize.z);
+                EditorGUILayout.PropertyField(so.FindProperty(nameof(GridSensorComponent.m_RootReference)), true);
             }
             EditorGUI.EndDisabledGroup();
             EditorGUILayout.PropertyField(so.FindProperty(nameof(GridSensorComponent.m_RotateWithAgent)), true);

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

         }
 
         [HideInInspector, SerializeField]
+        internal GameObject m_RootReference;
+        /// <summary>
+        /// Rotate the grid based on the direction the agent is facing.
+        /// </summary>
+        public GameObject RootReference
+        {
+            get { return (m_RootReference == null ? gameObject : m_RootReference); }
+            set { m_RootReference = value; }
+        }
+
+        [HideInInspector, SerializeField]
         internal string[] m_DetectableTags;
         /// <summary>
         /// List of tags that are detected.
                 m_GridSize,
                 m_RotateWithAgent,
                 m_ColliderMask,
-                gameObject,
+                RootReference,
                 m_DetectableTags,
                 m_InitialColliderBufferSize,
                 m_MaxColliderBufferSize