dynamically set session_key for session_ids in NDCG (pytorch#1658)

Summary:
Pull Request resolved: pytorch#1658

Previously session_key was hardcoded to "session_id" which is unsuitable as all models do not have the same session_key. Now the defined session_key from config will be used as the key instead.

Reviewed By: howei

Differential Revision: D53067426

fbshipit-source-id: e695b5c882eedcb6737168a8f585fbcdf427d590

torchrec/metrics/ndcg.py

@@ -307,4 +307,6 @@ def __init__(
             process_group=process_group,
             **kwargs,
         )
-        self._required_inputs.add("session_id")
+        # session_key is set through front end config
+        # pyre-ignore[6]
+        self._required_inputs.add(kwargs["session_key"])