change print to logging (#457)

* change print to logging

* always use logging.info

* use f-strings

* update code style

* set logging configs

* remove unused code

pytorch_lightning/root_module/memory.py

@@ -8,6 +8,7 @@ import subprocess
 import numpy as np
 import pandas as pd
 import torch
+import logging
 
 
 class ModelSummary(object):
@@ -166,7 +167,7 @@ def print_mem_stack():  # pragma: no cover
     for obj in gc.get_objects():
         try:
             if torch.is_tensor(obj) or (hasattr(obj, 'data') and torch.is_tensor(obj.data)):
-                print(type(obj), obj.size())
+                logging.info(type(obj), obj.size())
         except Exception:
             pass