Update delta tracker to inherit from model delta tracker (#3542)

Summary:
Pull Request resolved: #3542

internal
Update DeltaTracker to extend from Generic Class ModelDeltaTracker

General Context: We are in the process of transition to a unified DeltaTracker and this is 8/n diffs representing  changes towards the transition.

Differential Revision: D80615741

fbshipit-source-id: 748509a6a2d15581eaf3e82db7ed0163717c3e2d

Author: aliafzal

torchrec/distributed/model_tracker/model_delta_tracker.py

@@ -132,7 +132,6 @@ def clear(self, consumer: Optional[str] = None) -> None:
         """
         pass
 
-    @abstractmethod
     def step(self) -> None:
         """
         Advance the batch index for all consumers.