use yaml.safe_load instead of yaml.load (#2124)

ml-agents/mlagents/trainers/learn.py

 def load_config(trainer_config_path):
     try:
         with open(trainer_config_path) as data_file:
-            trainer_config = yaml.load(data_file)
+            trainer_config = yaml.safe_load(data_file)
             return trainer_config
     except IOError:
         raise UnityEnvironmentException(

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

 
 @pytest.fixture
 def dummy_config():
-    return yaml.load(
+    return yaml.safe_load(
         """
             hidden_units: 128
             learning_rate: 3.0e-4

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

 
 @pytest.fixture
 def dummy_config():
-    return yaml.load(
+    return yaml.safe_load(
         """
         trainer: ppo
         batch_size: 32

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

 
 @pytest.fixture
 def dummy_config():
-    return yaml.load(
+    return yaml.safe_load(
         """
         default:
             trainer: ppo
 
 @pytest.fixture
 def dummy_online_bc_config():
-    return yaml.load(
+    return yaml.safe_load(
         """
         default:
             trainer: online_bc
 
 @pytest.fixture
 def dummy_offline_bc_config():
-    return yaml.load(
+    return yaml.safe_load(
         """
         default:
             trainer: offline_bc
 
 @pytest.fixture
 def dummy_bad_config():
-    return yaml.load(
+    return yaml.safe_load(
         """
         default:
             trainer: incorrect_trainer