Enable the switching of scene to control mode, load the corresponding scene using environment variable (#1956)

* Added the builder script

* Removed the menu item

* Changed the brainToControl to public

* Added the scene for switching

* Modified according to the comments

* Removed the Builder and BuilderUtils script, made all of the logic into the Startup.cs

* Switched back to the previous way using PreExport method

* Added the return at the EOF.

* Resolved the codacy comments.

* Removed one empty line

* Resolved the 2 round comments

UnitySDK/Assets/ML-Agents/Editor/Builder.cs

+#if UNITY_CLOUD_BUILD
+
+namespace MLAgents
+{
+    public static class Builder
+    {
+        public static void PreExport()
+        {
+            BuilderUtils.SwitchAllLearningBrainToControlMode();
+        }
+    }
+}
+
+#endif

UnitySDK/Assets/ML-Agents/Editor/Builder.cs.meta

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UnitySDK/Assets/ML-Agents/Editor/BuilderUtils.cs

+#if UNITY_CLOUD_BUILD
+
+using System.Linq;
+using UnityEditor;
+using UnityEditor.SceneManagement;
+using UnityEngine;
+using System.IO;
+
+namespace MLAgents
+{
+    public static class BuilderUtils
+    {
+        public static void SwitchAllLearningBrainToControlMode()
+        {
+            Debug.Log("The Switching to control mode function is triggered");
+            string[] scenePaths = Directory.GetFiles("Assets/ML-Agents/Examples/", "*.unity", SearchOption.AllDirectories);
+            foreach (string scenePath in scenePaths)
+            {
+                var curScene = EditorSceneManager.OpenScene(scenePath);
+                var aca = SceneAsset.FindObjectOfType<Academy>();
+                if (aca != null)
+                {
+                    var learningBrains = aca.broadcastHub.broadcastingBrains.Where(
+                        x => x != null && x is LearningBrain);
+                    foreach (Brain brain in learningBrains)
+                    {
+                        if (!aca.broadcastHub.IsControlled(brain))
+                        {
+                            Debug.Log("Switched brain in scene " + scenePath);
+                            aca.broadcastHub.SetControlled(brain, true);
+                        }
+                    }
+                    EditorSceneManager.SaveScene(curScene);
+                }
+                else
+                {
+                    Debug.Log("scene " + scenePath + " doesn't have a Academy in it");
+                }
+            }
+        }
+    }
+}
+    
+#endif

UnitySDK/Assets/ML-Agents/Editor/BuilderUtils.cs.meta

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UnitySDK/Assets/ML-Agents/Examples/Startup.meta

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UnitySDK/Assets/ML-Agents/Scripts/Startup.cs

+using System;
+using System.Linq;
+using UnityEngine;
+using UnityEngine.SceneManagement;
+
+namespace MLAgents
+{
+    public class Startup: MonoBehaviour
+    {
+        void Awake()
+        {   
+            string sceneName = Environment.GetEnvironmentVariable("SCENE_NAME");
+            SwitchScene(sceneName);
+        }
+        
+        private static void SwitchScene(string sceneName)
+        {
+            if (sceneName == null)
+            {
+                throw new ArgumentException("You didn't specified the SCENE_NAME environment variable");
+            }
+            if (SceneUtility.GetBuildIndexByScenePath(sceneName) < 0)
+            {
+                throw new ArgumentException("The scene " + sceneName + " doesn't exist within your build. ");
+            }
+            SceneManager.LoadSceneAsync(sceneName);
+        }
+    }
+}

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