[tune] using None as the parameter default value instead of mutable dict (#1501)

* do not use dict as default parameter

* Update trial.py

python/ray/rllib/agent.py

@@ -63,7 +63,7 @@ class Agent(Trainable):
     _allow_unknown_subkeys = []
 
     def __init__(
-            self, config={}, env=None, registry=get_registry(),
+            self, config=None, env=None, registry=get_registry(),
             logger_creator=None):
         """Initialize an RLLib agent.
 
@@ -77,6 +77,8 @@ class Agent(Trainable):
                 object. If unspecified, a default logger is created.
         """
 
+        config = config or {}
+
         # Agents allow env ids to be passed directly to the constructor.
         self._env_id = env or config.get("env")
         Trainable.__init__(self, config, registry, logger_creator)