fix metric iterable checks

lm_eval/metrics.py

 import math
+from collections import Iterable
 from pprint import pprint
 
 import numpy as np
@@ -107,6 +108,10 @@ def ter(items):
     return sacrebleu.corpus_ter(preds, refs).score
 
 
+def is_non_str_iterable(obj):
+    return isinstance(obj, Iterable) and not isinstance(obj, str)
+
+
 def _sacreformat(refs, preds):
     """Format refs and preds for sacrebleu corpus calculation. It is very particular"""
     # Sacrebleu expects (List[str], List[List[str])
@@ -118,17 +123,17 @@ def _sacreformat(refs, preds):
 
     # We expect refs to be List[str] or List[List[str]], the outer list corresponding to preds
     # Must become List[List[str]] with the inner list corresponding to preds
-    if not isinstance(refs, list):
+    if not is_non_str_iterable(refs):
         refs = list(refs)
-    if not isinstance(refs[0], list):
+    if not is_non_str_iterable(refs):
         refs = [[ref] for ref in refs]
     refs = list(zip(*refs))
     # Note the number of refs in each ref list much match the number of preds
 
     # We expect preds to be List[str] or List[List[str]]. Must become List[str]
-    if not isinstance(preds, list):
+    if not is_non_str_iterable(preds):
         preds = list(preds)
-    if isinstance(preds[0], list):
+    if is_non_str_iterable(preds[0]):
         assert len(preds[0]) == 1, f"Pred must be a str, was {preds[0]}"
         preds = [pred[0] for pred in preds]