make sure DefaultTrainerDict is pickle-able (#4842)

com.unity.ml-agents/CHANGELOG.md

 #### com.unity.ml-agents (C#)
 - Fix a compile warning about using an obsolete enum in `GrpcExtensions.cs`. (#4812)
 #### ml-agents / ml-agents-envs / gym-unity (Python)
+- Fixed a bug that would cause an exception when `RunOptions` was deserialized via `pickle`. (#4842)
 
 
 ## [1.7.2-preview] - 2020-12-22

ml-agents/mlagents/trainers/settings.py

 
     class DefaultTrainerDict(collections.defaultdict):
         def __init__(self, *args):
-            super().__init__(TrainerSettings, *args)
+            # Depending on how this is called, args may have the defaultdict
+            # callable at the start of the list or not. In particular, unpickling
+            # will pass [TrainerSettings].
+            if args and args[0] == TrainerSettings:
+                super().__init__(*args)
+            else:
+                super().__init__(TrainerSettings, *args)
 
         def __missing__(self, key: Any) -> "TrainerSettings":
             if TrainerSettings.default_override is not None:

ml-agents/mlagents/trainers/tests/test_settings.py

 import attr
 import cattr
+import pickle
 import pytest
 import yaml
 
     test1_settings.max_steps = 1
     test1_settings.network_settings.hidden_units == default_settings_cls.network_settings.hidden_units
     check_if_different(test1_settings, default_settings_cls)
+
+
+def test_pickle():
+    # Make sure RunOptions is pickle-able.
+    run_options = RunOptions()
+    p = pickle.dumps(run_options)
+    pickle.loads(p)