[WIP] [Fix] Fixing collect observation called on done (#5375)

* [WIP] [Fix] Fixing collect observation called on done

* Update com.unity.ml-agents/Runtime/Agent.cs

* ⚠️ Modifying the test of stacking sensor when the agent is done

* modifying the documentation for BufferSensor to specify to call AddObservation in the CollectObservations method

com.unity.ml-agents/Runtime/Agent.cs

             m_Info.done = true;
             m_Info.maxStepReached = doneReason == DoneReason.MaxStepReached;
             m_Info.groupId = m_GroupId;
-            if (collectObservationsSensor != null)
+            UpdateSensors();
+            // Make sure the latest observations are being passed to training.
+            using (m_CollectObservationsChecker.Start())
-                // Make sure the latest observations are being passed to training.
-                collectObservationsSensor.Reset();
-                using (m_CollectObservationsChecker.Start())
-                {
-                    CollectObservations(collectObservationsSensor);
-                }
+                CollectObservations(collectObservationsSensor);
             }
             // Request the last decision with no callbacks
             // We request a decision so Python knows the Agent is done immediately

com.unity.ml-agents/Tests/Runtime/Sensor/StackingSensorTests.cs

                 agent1.RequestDecision();
                 aca.EnvironmentStep();
             }
-
-            policy.OnRequestDecision = () => SensorTestHelper.CompareObservation(sensor, new[] { 18f, 19f, 21f });
+            SensorTestHelper.CompareObservation(sensor, new[] { 18f, 19f, 20f });
+            policy.OnRequestDecision = () => SensorTestHelper.CompareObservation(sensor, new[] { 19f, 20f, 21f });
+            policy.OnRequestDecision = () => { };
+            for (int i = 0; i < 20; i++)
+            {
+                agent1.RequestDecision();
+                aca.EnvironmentStep();
+                SensorTestHelper.CompareObservation(sensor, new[] { Math.Max(0, i - 1f), i, i + 1 });
+            }
         }
 
         [Test]

docs/Learning-Environment-Design-Agents.md

  the `BufferSensor` will be able to collect.
 
 To add an entity's observations to a `BufferSensorComponent`, you need
-to call `BufferSensorComponent.AppendObservation()`
+to call `BufferSensorComponent.AppendObservation()` in the
+Agent.CollectObservations() method
 with a float array of size `Observation Size` as argument.
 
 __Note__: Currently, the observations put into the `BufferSensor` are
 #### Variable Length Observation Summary & Best Practices
  - Attach `BufferSensorComponent` to use.
- - Call `BufferSensorComponent.AppendObservation()` to add the observations
+ - Call `BufferSensorComponent.AppendObservation()` in the
+ Agent.CollectObservations() methodto add the observations
  of an entity to the `BufferSensor`.
  - Normalize the entities observations before feeding them into the `BufferSensor`.