[MLA-1742] backport SideChannel GC reduction (#4915)

com.unity.ml-agents/CHANGELOG.md

 
 ### Bug Fixes
 #### com.unity.ml-agents (C#)
+- Removed unnecessary memory allocations in `SensorShapeValidator.ValidateSensors()` (#4915)
+- Removed unnecessary memory allocations in `SideChannelManager.GetSideChannelMessage()` (#4915)
 
 
 ## [1.0.6] - 2020-11-13

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

             {
                 // Check for compatibility with the other Agents' Sensors
                 // TODO make sure this only checks once per agent
-                Debug.Assert(m_SensorShapes.Count == sensors.Count, $"Number of Sensors must match. {m_SensorShapes.Count} != {sensors.Count}");
+                Debug.AssertFormat(
+                    m_SensorShapes.Count == sensors.Count,
+                    "Number of Sensors must match. {0} != {1}",
+                    m_SensorShapes.Count,
+                    sensors.Count
+                );
                 for (var i = 0; i < Mathf.Min(m_SensorShapes.Count, sensors.Count); i++)
                 {
                     var cachedShape = m_SensorShapes[i];

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

         /// <returns></returns>
         internal static byte[] GetSideChannelMessage(Dictionary<Guid, SideChannel> sideChannels)
         {
+            if (!HasOutgoingMessages(sideChannels))
+            {
+                // Early out so that we don't create the MemoryStream or BinaryWriter.
+                // This is the most common case.
+                return Array.Empty<byte>();
+            }
+
             using (var memStream = new MemoryStream())
             {
                 using (var binaryWriter = new BinaryWriter(memStream))
                     return memStream.ToArray();
                 }
             }
+        }
+
+        /// <summary>
+        /// Check whether any of the sidechannels have queued messages.
+        /// </summary>
+        /// <param name="sideChannels"></param>
+        /// <returns></returns>
+        static bool HasOutgoingMessages(Dictionary<Guid, SideChannel> sideChannels)
+        {
+            foreach (var sideChannel in sideChannels.Values)
+            {
+                var messageList = sideChannel.MessageQueue;
+                if (messageList.Count > 0)
+                {
+                    return true;
+                }
+            }
+
+            return false;
         }
 
         /// <summary>