Put @keras_serializable only on layers it works on

And only run the test on TF*MainLayer classes so marked.

src/transformers/modeling_tf_utils.py

@@ -71,6 +71,7 @@ def keras_serializable(cls):
 
         cls.get_config = get_config
 
+    cls._keras_serializable = True
     return tf.keras.utils.register_keras_serializable()(cls)