old_contents: from .base import LightningLoggerBase, rank_zero_only
  from .test_tube_logger import TestTubeLogger
  
  try:
      from .mlflow_logger import MLFlowLogger
  except ModuleNotFoundError:
      pass
  
new_contents: from .base import LightningLoggerBase, rank_zero_only
  
  try:
      from .test_tube_logger import TestTubeLogger
  except ModuleNotFoundError:
      pass
  try:
      from .mlflow_logger import MLFlowLogger
  except ModuleNotFoundError:
      pass
  
subject: Make test_tube optional (#274)