Added ChrF prefix to the return output keys.

Sometimes when combining evaluation metrics `evaluate.combine(['bleu', 'chrf']`, without the prefixes, it's rather confusing, e.g. https://www.kaggle.com/code/alvations/huggingface-evaluate-for-mt-evaluations

chrf.py

@@ -181,8 +181,8 @@ class ChrF(evaluate.Metric):
         output = sb_chrf.corpus_score(predictions, transformed_references)
 
         return {
-            "score": output.score,
-            "char_order": output.char_order,
-            "word_order": output.word_order,
-            "beta": output.beta,
+            "chrf_score": output.score,
+            "chrf_char_order": output.char_order,
+            "chrf_word_order": output.word_order,
+            "chrf_beta": output.beta,
         }