Make IActuator implement IHeuristicProvider. (#5110)

com.unity.ml-agents.extensions/Runtime/Input/InputActionActuator.cs

     /// <see cref="Agent"/>'s <see cref="BehaviorParameters"/> indicate that the Agent is running in Heuristic Mode,
     /// this Actuator will write actions from the <see cref="InputSystem"/> to the <see cref="ActionBuffers"/> object.
     /// </summary>
-    public class InputActionActuator : IActuator, IHeuristicProvider, IBuiltInActuator
+    public class InputActionActuator : IActuator, IBuiltInActuator
     {
         readonly BehaviorParameters m_BehaviorParameters;
         readonly InputAction m_Action;

com.unity.ml-agents/CHANGELOG.md

 - The interface for disabling discrete actions in `IDiscreteActionMask` has changed.
 `WriteMask(int branch, IEnumerable<int> actionIndices)` was replaced with
 `SetActionEnabled(int branch, int actionIndex, bool isEnabled)`. See the
+- IActuator now implements IHeuristicProvider. (#5110)
 [Migration Guide](https://github.com/Unity-Technologies/ml-agents/blob/release_14_docs/docs/Migrating.md) for more
 details. (#5060)
 #### ml-agents / ml-agents-envs / gym-unity (Python)

com.unity.ml-agents/Runtime/Actuators/ActuatorManager.cs

                         discreteStart,
                         numDiscreteActions);
                 }
-
-                var heuristic = actuator as IHeuristicProvider;
-                heuristic?.Heuristic(new ActionBuffers(continuousActions, discreteActions));
+                actuator.Heuristic(new ActionBuffers(continuousActions, discreteActions));
                 continuousStart += numContinuousActions;
                 discreteStart += numDiscreteActions;
             }

com.unity.ml-agents/Runtime/Actuators/IActuator.cs

     /// <summary>
     /// Abstraction that facilitates the execution of actions.
     /// </summary>
-    public interface IActuator : IActionReceiver
+    public interface IActuator : IActionReceiver, IHeuristicProvider
     {
         /// <summary>
         /// The specification of the actions for this IActuator.

com.unity.ml-agents/Runtime/Actuators/VectorActuator.cs

     /// <summary>
     /// IActuator implementation that forwards calls to an <see cref="IActionReceiver"/> and an <see cref="IHeuristicProvider"/>.
     /// </summary>
-    internal class VectorActuator : IActuator, IHeuristicProvider, IBuiltInActuator
+    internal class VectorActuator : IActuator, IBuiltInActuator
     {
         IActionReceiver m_ActionReceiver;
         IHeuristicProvider m_HeuristicProvider;

docs/Migrating.md

     actionMask.SetActionEnabled(branch, 3, false);
 }
 ```
+- The `IActuator` interface now implements `IHeuristicProvider`.  Please add the corresponding `Heuristic(in ActionBuffers)`
+method to your custom Actuator classes.
 
 - The `ISensor.GetObservationShape()` method was removed, and `GetObservationSpec()` was added. You can use
 `ObservationSpec.Vector()` or `ObservationSpec.Visual()` to generate `ObservationSpec`s that are equivalent to