don't call evaluate.load() every time

lm_eval/api/metrics.py

@@ -136,14 +136,17 @@ def acc_mutual_info_fn(items):  # This is a passthrough function
     return items
 
 
+exact_match = evaluate.load("exact_match")
+
+
 @register_metric(
     metric="exact_match",
     higher_is_better=True,
     output_type="generate_until",
     aggregation="mean",
 )
-def exact_match_fn(**kwargs):  # This is a passthrough function
-    return evaluate.load("exact_match").compute(**kwargs)
+def exact_match_fn(**kwargs):
+    return exact_match.compute(**kwargs)
 
 
 @register_metric(