Removed the default for width and height of the executable training. (#3867)

* Removed the default for width and height of the executable training. This is to help relove #3835 since setting the screen resolution on Linux 2019.3 can cause issues.

* Editing the changelog

* Making fields in EngineConfig optional

com.unity.ml-agents/CHANGELOG.md

 - The maximum compatible version of tensorflow was changed to allow tensorflow 2.1 and 2.2. This
 will allow use with python 3.8 using tensorflow 2.2.0rc3.
 - `UnityRLCapabilities` was added to help inform users when RL features are mismatched between C# and Python packages. (#3831)
+- `mlagents-learn` will no longer set the width and height of the executable window to 84x84 when no width nor height arguments are given
 
 ### Bug Fixes
 

ml-agents-envs/mlagents_envs/side_channel/engine_configuration_channel.py

 
 
 class EngineConfig(NamedTuple):
-    width: int
-    height: int
-    quality_level: int
-    time_scale: float
-    target_frame_rate: int
-    capture_frame_rate: int
+    width: Optional[int]
+    height: Optional[int]
+    quality_level: Optional[int]
+    time_scale: Optional[float]
+    target_frame_rate: Optional[int]
+    capture_frame_rate: Optional[int]
 
     @staticmethod
     def default_config():

ml-agents/mlagents/trainers/learn.py

     eng_conf = argparser.add_argument_group(title="Engine Configuration")
     eng_conf.add_argument(
         "--width",
-        default=84,
+        default=None,
         type=int,
         help="The width of the executable window of the environment(s) in pixels "
         "(ignored for editor training).",
-        default=84,
+        default=None,
         type=int,
         help="The height of the executable window of the environment(s) in pixels "
         "(ignored for editor training)",