Fixes package validation errors for side-channels-related classes. (#3502)

* Fixes package validation errors for side-channels-related classes.

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

 
 namespace MLAgents
 {
+    /// <summary>
+    /// Side channel that supports modifying attributes specific to the Unity Engine.
+    /// </summary>
+
+        /// <summary>
+        /// Initializes the side channel.
+        /// </summary>
+        /// <inheritdoc/>
         public override void OnMessageReceived(byte[] data)
         {
             using (var memStream = new MemoryStream(data))

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

 
 namespace MLAgents
 {
+    /// <summary>
+    /// Interface for managing a collection of float properties keyed by a string variable.
+    /// </summary>
     public interface IFloatProperties
     {
         /// <summary>
         IList<string> ListProperties();
     }
 
+    /// <summary>
+    /// Side channel that is comprised of a collection of float variables, represented by
+    /// <see cref="IFloatProperties"/>
+    /// </summary>
     public class FloatPropertiesChannel : SideChannel, IFloatProperties
     {
         Dictionary<string, float> m_FloatProperties = new Dictionary<string, float>();
+        /// <summary>
+        /// Initializes the side channel with the provided channel ID.
+        /// </summary>
+        /// <param name="channelId">ID for the side channel.</param>
         public FloatPropertiesChannel(Guid channelId = default(Guid))
         {
             if (channelId == default(Guid))
             }
         }
 
+        /// <inheritdoc/>
         public override void OnMessageReceived(byte[] data)
         {
             var kv = DeserializeMessage(data);
             }
         }
 
+        /// <inheritdoc/>
         public void SetProperty(string key, float value)
         {
             m_FloatProperties[key] = value;
             }
         }
 
+        /// <inheritdoc/>
         public float GetPropertyWithDefault(string key, float defaultValue)
         {
             if (m_FloatProperties.ContainsKey(key))
             }
         }
 
+        /// <inheritdoc/>
+        /// <inheritdoc/>
         public IList<string> ListProperties()
         {
             return new List<string>(m_FloatProperties.Keys);

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

 
 namespace MLAgents
 {
+    /// <summary>
+    /// Side channel for managing raw bytes of data. It is up to the clients of this side channel
+    /// to interpret the messages.
+    /// </summary>
     public class RawBytesChannel : SideChannel
     {
         List<byte[]> m_MessagesReceived = new List<byte[]>();
             ChannelId = channelId;
         }
 
+        /// <inheritdoc/>
         public override void OnMessageReceived(byte[] data)
         {
             m_MessagesReceived.Add(data);

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

 
 namespace MLAgents
 {
+    /// <summary>
+    /// Side channels provide an alternative mechanism of sending/receiving data from Unity
+    /// to Python that is outside of the traditional machine learning loop. ML-Agents provides
+    /// some specific implementations of side channels, but users can create their own.
+    /// </summary>
     public abstract class SideChannel
     {
         // The list of messages (byte arrays) that need to be sent to Python via the communicator.
         /// An int identifier for the SideChannel. Ensures that there is only ever one side channel
         /// of each type. Ensure the Unity side channels will be linked to their Python equivalent.
         /// </summary>
-        /// <returns> The integer identifier of the SideChannel</returns>
+        /// <returns> The integer identifier of the SideChannel.</returns>
         public Guid ChannelId{
             get;
             protected set;

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

         }
 
         [Test]
-        public void TestStoreInitalize()
+        public void TestStoreInitialize()
         {
             var fileSystem = new MockFileSystem();