add openbookqa generative

lm_eval/tasks/openbookqa/openbookqa_generative.yaml

+task: openbookqa_generative
+dataset_path: openbookqa
+dataset_name: main
+output_type: generate_until
+training_split: train
+validation_split: validation
+test_split: test
+doc_to_text: |-
+  Given the following question and four candidate answers (A, B, C and D), choose the best answer.
+
+  Question: {{question_stem}}
+  A. {{choices.text[0]}}
+  B. {{choices.text[1]}}
+  C. {{choices.text[2]}}
+  D. {{choices.text[3]}}
+  Your response should end with "The best answer is [the_answer_letter]" where the [the_answer_letter] is one of choice letters, A, B, C or D.
+doc_to_target: answerKey
+should_decontaminate: true
+filter_list:
+  - name: "strict-match"
+    filter:
+      - function: "multi_choice_regex"
+      - function: "take_first"
+gen_kwargs:
+  until: []
+  max_gen_toks: 10
+metric_list:
+  - metric: exact_match
+    aggregation: mean
+    higher_is_better: true
+metadata:
+  version: 1.0