Fixing package validation errors. (#3808)

* Fixing package validation errors.

This impacts our API as two public variables have been made private.

TODO: fix our CI to catch these automatically, per commit.

* Changelog changes.

com.unity.ml-agents/CHANGELOG.md

   communication between Unity and the Python process.
 - The obsolete `Agent` methods `GiveModel`, `Done`, `InitializeAgent`,
   `AgentAction` and `AgentReset` have been removed.
-- The GhostTrainer has been extended to support asymmetric games and the asymmetric example environment Strikers Vs. Goalie has been added.
+- The GhostTrainer has been extended to support asymmetric games and the
+  asymmetric example environment Strikers Vs. Goalie has been added.
+- CameraSensorComponent.m_Grayscale and RenderTextureSensorComponent.m_Grayscale
+  were changed from `public` to `private` (#3808).
 
 ### Minor Changes
 

com.unity.ml-agents/Editor/EditorUtilities.cs

 
 namespace MLAgents.Editor
 {
+    /// <summary>
+    /// A static helper class for the Editor components of the ML-Agents SDK.
+    /// </summary>
     public static class EditorUtilities
     {
         /// <summary>

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

         /// <summary>
         /// When the Agent uses Heuristics, it will call this method every time it
         /// needs an action. This can be used for debugging or controlling the agent
-        /// with keyboard.
+        /// with keyboard. This can also be useful to record demonstrations for imitation learning.
-        /// <returns> A float array corresponding to the next action of the Agent
-        /// </returns>
+        /// <param name="actionsOut">An array corresponding to the next action of the Agent</param>
         public virtual void Heuristic(float[] actionsOut)
         {
             Debug.LogWarning("Heuristic method called but not implemented. Returning placeholder actions.");

com.unity.ml-agents/Runtime/Policies/BrainParameters.cs

         /// </summary>
         public SpaceType vectorActionSpaceType = SpaceType.Discrete;
 
+        /// <summary>
+        /// The number of actions specified by this Brain.
+        /// </summary>
         public int numActions
         {
             get

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

         }
 
         [HideInInspector, SerializeField, FormerlySerializedAs("grayscale")]
-        public bool m_Grayscale;
+        bool m_Grayscale;
 
         /// <summary>
         /// Whether to generate grayscale images or color.

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

         [HideInInspector, SerializeField, FormerlySerializedAs("renderTexture")]
         RenderTexture m_RenderTexture;
 
+        /// <summary>
+        /// Stores the <see cref="RenderTexture"/> associated with this sensor.
+        /// </summary>
         public RenderTexture renderTexture
         {
             get { return m_RenderTexture;  }
         }
 
         [HideInInspector, SerializeField, FormerlySerializedAs("grayscale")]
-        public bool m_Grayscale;
+        bool m_Grayscale;
 
         /// <summary>
         /// Whether the RenderTexture observation should be converted to grayscale or not.

com.unity.ml-agents/Runtime/SideChannels/FloatPropertiesChannel.cs

 namespace MLAgents.SideChannels
 {
     /// <summary>
-    /// Side channel that is comprised of a collection of float variables, represented by
-    /// <see cref="IFloatProperties"/>
+    /// Side channel that is comprised of a collection of float variables.
     /// </summary>
     public class FloatPropertiesChannel : SideChannel
     {
             action?.Invoke(value);
         }
 
-        /// <inheritdoc/>
         public float GetPropertyWithDefault(string key, float defaultValue)
         {
             float valueOut;
 
-        /// <inheritdoc/>
-        /// <inheritdoc/>
         public IList<string> ListProperties()
         {
             return new List<string>(m_FloatProperties.Keys);

com.unity.ml-agents/Runtime/SideChannels/SideChannel.cs

         /// <summary>
         /// Queues a message to be sent to Python during the next simulation step.
         /// </summary>
-        /// <param name="data"> The byte array of data to be sent to Python.</param>
+        /// <param name="msg"> The byte array of data to be sent to Python.</param>
         protected void QueueMessageToSend(OutgoingMessage msg)
         {
             MessageQueue.Add(msg.ToByteArray());

com.unity.ml-agents/Runtime/SideChannels/SideChannelUtils.cs

 
 namespace MLAgents.SideChannels
 {
+    /// <summary>
+    /// Collection of static utilities for managing the registering/unregistering of
+    /// <see cref="SideChannels"/> and the sending/receiving of messages for all the channels.
+    /// </summary>
     public static class SideChannelUtils
     {