docstrings and cleanup around actuators (#4467)

* docstrings and cleanup around actuators

* move ActionSpec property from IActionReceiver to IActuator

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

         public static ActionBuffers FromDiscreteActions(float[] discreteActions)
         {
             return new ActionBuffers(ActionSegment<float>.Empty, discreteActions == null ? ActionSegment<int>.Empty
-                                : new ActionSegment<int>(Array.ConvertAll(discreteActions,
-                                    x => (int)x)));
+                : new ActionSegment<int>(Array.ConvertAll(discreteActions,
+                    x => (int)x)));
+        /// <summary>
+        /// Construct an <see cref="ActionBuffers"/> instance with the continuous and discrete actions that will
+        /// be used.
+        /// /// </summary>
+        /// <param name="continuousActions">The continuous actions to send to an <see cref="IActionReceiver"/>.</param>
+        /// <param name="discreteActions">The discrete actions to send to an <see cref="IActionReceiver"/>.</param>
         public ActionBuffers(float[] continuousActions, int[] discreteActions)
             : this(new ActionSegment<float>(continuousActions), new ActionSegment<int>(discreteActions)) { }
 
     /// </summary>
     public interface IActionReceiver
     {
-
-        /// <summary>
-        /// The specification of the Action space for this IActionReceiver.
-        /// </summary>
-        /// <seealso cref="ActionSpec"/>
-        ActionSpec ActionSpec { get; }
-
         /// <summary>
         /// Method called in order too allow object to execute actions based on the
         /// <see cref="ActionBuffers"/> contents.  The structure of the contents in the <see cref="ActionBuffers"/>
         /// </remarks>
         /// <seealso cref="IActionReceiver.OnActionReceived"/>
         void WriteDiscreteActionMask(IDiscreteActionMask actionMask);
-    }
-
-    /// <summary>
-    /// Helper methods to be shared by all classes that implement <see cref="IActionReceiver"/>.
-    /// </summary>
-    public static class ActionReceiverExtensions
-    {
-        /// <summary>
-        /// Returns the number of discrete branches + the number of continuous actions.
-        /// </summary>
-        /// <param name="actionReceiver"></param>
-        /// <returns></returns>
-        public static int TotalNumberOfActions(this IActionReceiver actionReceiver)
-        {
-            return actionReceiver.ActionSpec.NumContinuousActions + actionReceiver.ActionSpec.NumDiscreteActions;
-        }
     }
 }

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

-using System;
-using UnityEngine;
-
 namespace Unity.MLAgents.Actuators
 {
     /// <summary>
     {
         /// <summary>
+        /// The specification of the Action space for this IActuator.
+        /// </summary>
+        /// <seealso cref="ActionSpec"/>
+        ActionSpec ActionSpec { get; }
+
+        /// <summary>
+        /// <summary>
+        /// Resets the internal state of the actuator. This is called at the end of an Agent's episode.
+        /// Most implementations can leave this empty.
+        /// </summary>
+    }
+
+    /// <summary>
+    /// Helper methods to be shared by all classes that implement <see cref="IActuator"/>.
+    /// </summary>
+    public static class IActuatorExtensions
+    {
+        /// <summary>
+        /// Returns the number of discrete branches + the number of continuous actions.
+        /// </summary>
+        /// <param name="actuator"></param>
+        /// <returns></returns>
+        public static int TotalNumberOfActions(this IActuator actuator)
+        {
+            return actuator.ActionSpec.NumContinuousActions + actuator.ActionSpec.NumDiscreteActions;
+        }
     }
 }

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

 
 namespace Unity.MLAgents.Actuators
 {
-    public class VectorActuator : IActuator
+    /// <summary>
+    /// IActuator implementation that forwards to an <see cref="IActionReceiver"/>.
+    /// </summary>
+    internal class VectorActuator : IActuator
     {
         IActionReceiver m_ActionReceiver;
 
             private set => m_ActionBuffers = value;
         }
 
+        /// <summary>
+        /// Create a VectorActuator that forwards to the provided IActionReceiver.
+        /// </summary>
+        /// <param name="actionReceiver">The <see cref="IActionReceiver"/> used for OnActionReceived and WriteDiscreteActionMask.</param>
+        /// <param name="vectorActionSize">For discrete action spaces, the branch sizes for each action.
+        /// For continuous action spaces, the number of actions is the 0th element.</param>
+        /// <param name="spaceType"></param>
+        /// <param name="name"></param>
+        /// <exception cref="ArgumentOutOfRangeException">Thrown for invalid <see cref="SpaceType"/></exception>
         public VectorActuator(IActionReceiver actionReceiver,
                               int[] vectorActionSize,
                               SpaceType spaceType,

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

             EndEpisodeAndReset(DoneReason.MaxStepReached);
         }
 
-
         /// <summary>
         /// Internal method to end the episode and reset the Agent.
         /// </summary>
             }
             CollectDiscreteActionMasks(m_ActionMasker);
         }
-
-        ActionSpec IActionReceiver.ActionSpec { get; }
 
         /// <summary>
         /// Implement `OnActionReceived()` to specify agent behavior at every step, based

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

         void Update();
 
         /// <summary>
-        /// Resets the internal states of the sensor. This is called at the end of an Agent's episode.
+        /// Resets the internal state of the sensor. This is called at the end of an Agent's episode.
         /// Most implementations can leave this empty.
         /// </summary>
         void Reset();