remove optional comment in docstring for step (#482)

Summary:
Pull Request resolved: #482

received this error:

```
[0]: > self.tb_logger.log("avg_test_auc", self.total_auc_sum / self.num_eval_steps)
[0]:TypeError: TensorBoardLogger.log() missing 1 required positional argument: 'step'
```

Docstring says it is optional, but it is typed to be enforced. Removing `optional` from docstring

Reviewed By: ananthsub

Differential Revision: D47921178

fbshipit-source-id: a1c650d34ef23271daf34153825d62341c3be16f

torchtnt/utils/loggers/tensorboard.py

@@ -93,7 +93,7 @@ def log_dict(self, payload: Mapping[str, Scalar], step: int) -> None:
 
         Args:
             payload (dict): dictionary of tag name and scalar value
-            step (int, Optional): step value to record
+            step (int): step value to record
         """
 
         if self._writer:
@@ -106,7 +106,7 @@ def log(self, name: str, data: Scalar, step: int) -> None:
         Args:
             name (string): tag name used to group scalars
             data (float/int/Tensor): scalar data to log
-            step (int, optional): step value to record
+            step (int): step value to record
         """
 
         if self._writer: