Make test_tube optional (#274)

pytorch_lightning/logging/__init__.py

@@ -1,6 +1,9 @@
 from .base import LightningLoggerBase, rank_zero_only
-from .test_tube_logger import TestTubeLogger
 
+try:
+    from .test_tube_logger import TestTubeLogger
+except ModuleNotFoundError:
+    pass
 try:
     from .mlflow_logger import MLFlowLogger
 except ModuleNotFoundError: