fix the regex string in mmlu_pro template (#2238)

* fix the regex string in yaml file

* Update samplers.py

---------
Co-authored-by: Lintang Sutawika <lintang@eleuther.ai>

lm_eval/api/samplers.py

@@ -82,8 +82,9 @@ class ContextSampler:
                 if self.config.doc_to_choice is None or isinstance(doc_content, str)
                 else self.doc_to_choice(doc)[doc_content]
             )
-            labeled_examples += self.target_delimiter
+
             if doc_target != "":
+                labeled_examples += self.target_delimiter
                 labeled_examples += (
                     str(doc_target[0])
                     if isinstance(doc_target, list)

lm_eval/tasks/mmlu_pro/_default_template_yaml

@@ -12,7 +12,7 @@ filter_list:
   - name: "custom-extract"
     filter:
       - function: "regex"
-        regex_pattern: r"answer is \(?([ABCDEFGHIJ])\)?"
+        regex_pattern: 'answer is \(?([ABCDEFGHIJ])\)?'
         # regex_pattern: r".*[aA]nswer:\s*([A-J])",
       - function: "take_first"
 generation_kwargs: