fix duplicate mask in f1_score (pytorch#145)

Summary: Pull Request resolved: pytorch#145

Reviewed By: ananthsub

Differential Revision: D45188253

fbshipit-source-id: 6b29497cb849af78f80ea8aefdf7585a9476ac8a

Author: JKSenthil

torcheval/metrics/functional/classification/f1_score.py

@@ -227,8 +227,7 @@ def _f1_score_compute(
     elif average == "macro":
         return f1.mean()
     elif average == "weighted":
-        # pyre-fixme[61]: `mask` is undefined, or not always defined.
-        return (f1 * (num_label[mask] / num_label.sum())).sum()
+        return (f1 * (num_label / num_label.sum())).sum()
     else:  # average is None
         return f1