fix deleted observations (#4366)

Project/Assets/ML-Agents/Examples/3DBall/Scripts/Ball3DAgent.cs

     public override void CollectObservations(VectorSensor sensor)
     {
         sensor.AddObservation(gameObject.transform.rotation.z);
+        sensor.AddObservation(gameObject.transform.rotation.x);
+        sensor.AddObservation(ball.transform.position - gameObject.transform.position);
+        sensor.AddObservation(m_BallRb.velocity);
-    
+
     public override void OnActionReceived(ActionBuffers actionBuffers)
     {
         var actionZ = 2f * Mathf.Clamp(actionBuffers.ContinuousActions[0], -1f, 1f);