add openbookqa config

lm_eval/tasks/openbookqa/openbookqa.yaml

+group:
+  - multiple_choice
+task: openbookqa
+dataset_path: openbookqa
+dataset_name: main
+output_type: multiple_choice
+training_split: train
+validation_split: validation
+test_split: test
+template_aliases: "{% set answer_choices = choices['text'] %}{% set gold = choices.label.index(answerKey.lstrip()) %}" # set the list of possible answer choices, and set what this doc's gold answer is (set what ds column used, and what)
+doc_to_text: "{{question_stem}}"
+doc_to_target: "{{gold}}" # this will be cast to an int.
+should_decontaminate: true
+doc_to_decontamination_query: "{{question_stem}}"
+metric_list:
+  - metric: acc
+    aggregation: mean
+    higher_is_better: true
+  - metric: acc_norm
+    aggregation: mean
+    higher_is_better: true