add default back for session id in NDCG (pytorch#1677)

Summary:
Pull Request resolved: pytorch#1677

Add default back to "session_id" if session_key is not set in frontend config.

Reviewed By: fandw06

Differential Revision: D53326155

fbshipit-source-id: 92f88cda589c3ee69003419de424c9c0a53a99b9

torchrec/metrics/ndcg.py

@@ -307,6 +307,9 @@ def __init__(
             process_group=process_group,
             **kwargs,
         )
-        # session_key is set through front end config
-        # pyre-ignore[6]
-        self._required_inputs.add(kwargs["session_key"])
+        # session_key is set through front end config, default back to "session_id" if does not exist
+        if "session_key" not in kwargs:
+            self._required_inputs.add(SESSION_KEY)
+        else:
+            # pyre-ignore[6]
+            self._required_inputs.add(kwargs["session_key"])