Fixes a TypeError for generative metrics. (#386)

Co-authored-by: Clémentine Fourrier <22726840+clefourrier@users.noreply.github.com>

src/lighteval/metrics/sample_preparator.py

@@ -55,6 +55,7 @@ class PerplexityCorpusMetricInput(CorpusMetricInput):
 
 
 class GenerativePreparator:
+    @staticmethod
     def prepare(golds: list[str], predictions: list[str], **kwargs):
         """Prepares an individual generative example to the format expected by metrics computed at the corpus level (aggregated).