remove unused attribute from iteration time logger (#792)

Summary: Pull Request resolved: #792

Reviewed By: JKSenthil

Differential Revision: D56329189

fbshipit-source-id: 7ce8cbb820110a942c410da6ab114a9477286d05

Author: galrotem

torchtnt/framework/callbacks/iteration_time_logger.py

@@ -7,7 +7,7 @@
 # pyre-strict
 
 
-from typing import cast, Optional, Union
+from typing import cast, Union
 
 from pyre_extensions import none_throws
 from torch.utils.tensorboard import SummaryWriter
@@ -31,8 +31,6 @@ class IterationTimeLogger(Callback):
         log_every_n_steps: an optional int to control the log frequency
     """
 
-    _writer: Optional[SummaryWriter] = None
-
     def __init__(
         self,
         logger: Union[MetricLogger, SummaryWriter],