fix: change model_dir to training job name if it is for tuning. (aws#179)

* Change model_dir to training job if the training job is for tuning.

Author: chuyang-deng

test/unit/test_training.py

@@ -295,4 +295,4 @@ def test_main_tuning_mpi_model_dir(configure_s3_env, read_hyperparameters, train
     training_env.return_value = single_machine_training_env
     os.environ['SAGEMAKER_REGION'] = REGION
     training.main()
-    configure_s3_env.assert_called_once_with('/opt/ml/model', REGION)
+    configure_s3_env.assert_called_once_with('/opt/ml/model', REGION)