Revert "small fix in perplexity runner"

This reverts commit b195416.

Author: jindrahelcl

neuralmonkey/runners/perplexity_runner.py

@@ -36,4 +36,4 @@ def fetches(self) -> Dict[str, tf.Tensor]:
 
     @property
     def loss_names(self) -> List[str]:
-        return ["xents"]
+        return ["xent"]