Add tests to make sure il2cpp works with fast training.

.yamato/standalone-build-test.yml

 {% metadata_file .yamato/test_versions.metafile %}
 ---
 {% for editor in test_editors %}
-test_linux_standalone_{{ editor.version }}:
-  name: Test Linux Standalone {{ editor.version }}
+{% for backend in scripting_backends %}
+test_linux_standalone_{{ editor.version }}_{{ backend.backend }}:
+  name: Test Linux Standalone {{ editor.version }} {{ backend.backend }}
   agent:
     type: Unity::VM
     image: package-ci/ubuntu:v1.3.1-719011
     - python3 -m pip install pyyaml --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
     - python3 -m pip install unity-downloader-cli --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple --upgrade
     - unity-downloader-cli -u {{ editor.version }} -c editor --wait --fast
-    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux
-    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/Examples/Basic/Scenes/Basic.unity
-    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/Examples/Match3/Scenes/Match3.unity
-    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/Examples/WallJump/Scenes/WallJump.unity
-    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/TestScenes/TestCompressedGrid/TestGridCompressed.unity
-    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/TestScenes/TestCompressedTexture/TestTextureCompressed.unity
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --mlagents-scripting-backend={{ backend.backend }}
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/Examples/Basic/Scenes/Basic.unity --mlagents-scripting-backend={{ backend.backend }}
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/Examples/Match3/Scenes/Match3.unity --mlagents-scripting-backend={{ backend.backend }}
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/Examples/WallJump/Scenes/WallJump.unity --mlagents-scripting-backend={{ backend.backend }}
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/TestScenes/TestCompressedGrid/TestGridCompressed.unity --mlagents-scripting-backend={{ backend.backend }}
+    - python3 -u -m ml-agents.tests.yamato.standalone_build_tests --build-target=linux --scene=Assets/ML-Agents/TestScenes/TestCompressedTexture/TestTextureCompressed.unity --mlagents-scripting-backend={{ backend.backend }}
   triggers:
     cancel_old_ci: true
     expression: |
         - "artifacts/testPlayer*/**"
         - "artifacts/**/UnityPlayer.so"
 {% endfor %}
+{% endfor %}

.yamato/test_versions.metafile

     extra_test: sensor
   - version: 2021.1
     extra_test: llapi
+scripting_backends:
+    - backend: mono
+    - backend: il2cpp

.yamato/training-int-tests.yml

 {% metadata_file .yamato/test_versions.metafile %}
 ---
 {% for editor in test_editors %}
-test_linux_training_int_{{ editor.version }}:
-  name: Test Linux Fast Training {{ editor.version }}
+{% for backend in scripting_backends %}
+test_linux_training_int_{{ editor.version }}_{{ backend.backend }}:
+  name: Test Linux Fast Training {{ editor.version }} {{ backend.backend }}
   agent:
     type: Unity::VM
     image: package-ci/ubuntu:v1.3.1-719011
       python -m pip install pyyaml --index-url https://artifactory.prd.it.unity3d.com/artifactory/api/pypi/pypi/simple
       python -u -m ml-agents.tests.yamato.training_int_tests
   dependencies:
-    - .yamato/standalone-build-test.yml#test_linux_standalone_{{ editor.version }}
+    - .yamato/standalone-build-test.yml#test_linux_standalone_{{ editor.version }}_{{ backend.backend }}
   triggers:
     cancel_old_ci: true
     expression: |
         - "artifacts/testPlayer*/**"
         - "artifacts/models/**"
 {% endfor %}
+{% endfor % }

Project/Assets/ML-Agents/Editor/Tests/StandaloneBuildTest.cs

     {
         const string k_OutputCommandLineFlag = "--mlagents-build-output-path";
         const string k_SceneCommandLineFlag = "--mlagents-build-scene-path";
-        private const string k_BuildTargetFlag = "--mlagents-build-target";
+        const string k_BuildTargetFlag = "--mlagents-build-target";
+        const string k_ScriptingBackendFlag = "--mlagents-scripting-backend";
 
         public static void BuildStandalonePlayerOSX()
         {
             var buildTarget = BuildTarget.StandaloneOSX;
+            var scriptingBackend = ScriptingImplementation.Mono2x;
 
             var args = Environment.GetCommandLineArgs();
             for (var i = 0; i < args.Length - 1; i++)
                 {
                     buildTarget = (BuildTarget)Enum.Parse(typeof(BuildTarget), args[i + 1], ignoreCase: true);
                 }
+                else if (args[i] == k_ScriptingBackendFlag)
+                {
+                    var val = args[i + 1];
+                    if (val == "mono")
+                    {
+                        scriptingBackend = ScriptingImplementation.Mono2x;
+                    }
+                    else
+                    {
+                        scriptingBackend = ScriptingImplementation.IL2CPP;
+                    }
+                }
+            PlayerSettings.SetScriptingBackend(BuildTargetGroup.Standalone, scriptingBackend);
             var buildResult = BuildPipeline.BuildPlayer(
                 scenes,
                 outputPath,

ml-agents/tests/yamato/standalone_build_tests.py

     parser.add_argument(
         "--build-target", default="mac", choices=["mac", "linux", "ios", "webgl"]
     )
+    parser.add_argument(
+        "--scripting-backen", default="mono", choices=["mono", "il2cpp"]
+    )
     args = parser.parse_args()
     main(args.scene, args.build_target)

ml-agents/tests/yamato/yamato_utils.py

     output_path: str = None,
     scene_path: str = None,
     build_target: str = None,
+    scripting_backend: str = "mono",
     log_output_path: Optional[str] = f"{get_base_output_path()}/standalone_build.txt",
 ) -> int:
     """
         test_args += ["--mlagents-build-scene-path", scene_path]
     if build_target_enum is not None:
         test_args += ["--mlagents-build-target", build_target_enum]
+    if scripting_backend != "mono":
+        test_args += ["--mlagents-scripting-backend", scripting_backend]
     print(f"{' '.join(test_args)} ...")
 
     timeout = 30 * 60  # 30 minutes, just in case