Remove RB sensor and add X Z velocity obs

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

   m_CellScale: {x: 1, y: 0.5, z: 1}
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+    VectorObservationSize: 2
+    NumStackedVectorObservations: 1
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-  sensorName: RBSensor
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Project/Assets/ML-Agents/Examples/PushBlock/Scripts/PushAgentCollab.cs

 using UnityEngine;
 using Unity.MLAgents;
 using Unity.MLAgents.Actuators;
+using Unity.MLAgents.Sensors;
 
 public class PushAgentCollab : Agent
 {
     {
         // Move the agent using the action.
         MoveAgent(actionBuffers.DiscreteActions);
+    }
+
+    /// <summary>
+    /// Called every time a decision is requested.
+    /// </summary>
+    public override void CollectObservations(VectorSensor sensor)
+    {
+        sensor.AddObservation(Vector3.Dot(m_AgentRb.velocity, m_AgentRb.transform.forward));
+        sensor.AddObservation(Vector3.Dot(m_AgentRb.velocity, m_AgentRb.transform.right));
     }
 
     public override void Heuristic(in ActionBuffers actionsOut)