access to observations in Heuristic (#3825)

* access to observations in Heuristic

* changelog

com.unity.ml-agents/CHANGELOG.md

 - Academy.InferenceSeed property was added. This is used to initialize the
   random number generator in ModelRunner, and is incremented for each ModelRunner. (#3823)
 - Updated Barracuda to 0.6.3-preview.
- - Model updates can now happen asynchronously with environment steps for better performance. (#3690)
- - `num_updates` and `train_interval` for SAC were replaced with `steps_per_update`. (#3690)
+- Added `Agent.GetObservations(), which returns a read-only view of the observations
+  added in `CollectObservations()`. (#3825)
+- Model updates can now happen asynchronously with environment steps for better performance. (#3690)
+- `num_updates` and `train_interval` for SAC were replaced with `steps_per_update`. (#3690)
 
 ### Bug Fixes
 

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

 using System;
 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using UnityEngine;
 using Barracuda;
 using MLAgents.Sensors;
         /// </remarks>
         public virtual void CollectObservations(VectorSensor sensor)
         {
+        }
+
+        /// <summary>
+        /// Returns a read-only view of the observations that were generated in
+        /// <see cref="CollectObservations(VectorSensor)"/>. This is mainly useful inside of a
+        /// <see cref="Heuristic(float[])"/> method to avoid recomputing the observations.
+        /// </summary>
+        /// <returns>A read-only view of the observations list.</returns>
+        public ReadOnlyCollection<float> GetObservations()
+        {
+            return collectObservationsSensor.GetObservations();
         }
 
         /// <summary>

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

 using System.Collections.Generic;
+using System.Collections.ObjectModel;
 using UnityEngine;
 
 namespace MLAgents.Sensors
             }
             adapter.AddRange(m_Observations);
             return expectedObservations;
+        }
+
+        /// <summary>
+        /// Returns a read-only view of the observations that added.
+        /// </summary>
+        /// <returns>A read-only view of the observations list.</returns>
+        internal ReadOnlyCollection<float> GetObservations()
+        {
+            return m_Observations.AsReadOnly();
         }
 
         /// <inheritdoc/>

com.unity.ml-agents/Tests/Editor/MLAgentsEditModeTest.cs

 
         public override void Heuristic(float[] actionsOut)
         {
+            var obs = GetObservations();
+            actionsOut[0] = obs[0];
             heuristicCalls++;
         }
     }
             Assert.AreEqual(numSteps, agent1.heuristicCalls);
             Assert.AreEqual(numSteps, agent1.sensor1.numWriteCalls);
             Assert.AreEqual(numSteps, agent1.sensor2.numCompressedCalls);
+
+            // Make sure the Heuristic method read the observation and set the action
+            Assert.AreEqual(agent1.collectObservationsCallsForEpisode, agent1.GetAction()[0]);
         }
     }