Move memory validation to settings

ml-agents/mlagents/trainers/policy/tf_policy.py

         if self.network_settings.memory is not None:
             self.m_size = self.network_settings.memory.memory_size
             self.sequence_length = self.network_settings.memory.sequence_length
-            if self.m_size == 0:
-                raise UnityPolicyException(
-                    "The memory size for brain {0} is 0 even "
-                    "though the trainer uses recurrent.".format(brain.brain_name)
-                )
-            elif self.m_size % 2 != 0:
-                raise UnityPolicyException(
-                    "The memory size for brain {0} is {1} "
-                    "but it must be divisible by 2.".format(
-                        brain.brain_name, self.m_size
-                    )
-                )
         self._initialize_tensorflow_references()
         self.load = load
 
     def _load_graph(self, model_path: str, reset_global_steps: bool = False) -> None:
         with self.graph.as_default():
             self.saver = tf.train.Saver(max_to_keep=self.keep_checkpoints)
-            logger.info(
-                "Loading model for brain {} from {}.".format(
-                    self.brain.brain_name, model_path
-                )
-            )
+            logger.info(f"Loading model from {model_path}.")
             ckpt = tf.train.get_checkpoint_state(model_path)
             if ckpt is None:
                 raise UnityPolicyException(

ml-agents/mlagents/trainers/settings.py

 
 @attr.s(auto_attribs=True)
 class NetworkSettings:
-    @attr.s(auto_attribs=True)
+    @attr.s
-        sequence_length: int = 64
-        memory_size: int = 128
+        sequence_length: int = attr.ib(default=64)
+        memory_size: int = attr.ib(default=128)
+
+        @memory_size.validator
+        def _check_valid_memory_size(self, attribute, value):
+            if value <= 0:
+                raise TrainerConfigError(
+                    "When using a recurrent network, memory size must be greater than 0."
+                )
+            elif value % 2 != 0:
+                raise TrainerConfigError(
+                    "When using a recurrent network, memory size must be divisible by 2."
+                )
 
     normalize: bool = False
     hidden_units: int = 128

ml-agents/mlagents/trainers/trainer/trainer.py

         Exports the model
         """
         policy = self.get_policy(name_behavior_id)
-        settings = SerializationSettings(policy.model_path, policy.brain.brain_name)
+        settings = SerializationSettings(policy.model_path, self.brain_name)
         export_policy_model(settings, policy.graph, policy.sess)
 
     @abc.abstractmethod