add modules to contrib handlers namespace

previously this occured as a side effect, adding it explicitly now.

ignite/contrib/handlers/__init__.py

@@ -19,3 +19,4 @@
 
 from ignite.handlers.visdom_logger import VisdomLogger
 from ignite.handlers.wandb_logger import WandBLogger
+from ignite.handlers import clearml_logger, mlflow_logger, neptune_logger, polyaxon_logger, tensorboard_logger, tqdm_logger, wandb_logger, visdom_logger