bug fix - add unique idx per question

mrqa.py

@@ -202,7 +202,7 @@ class MRQA(datasets.GeneratorBasedBuilder):
                 header = next(f)
                 subset = json.loads(header)["header"]["dataset"]
                 idx = 0
-                for row in enumerate(f):
+                for row in f:
                     paragraph = json.loads(row)
                     context = paragraph["context"].strip()
                     if subset == "HotpotQA":