add prost config

lm_eval/tasks/prost/corypaik_prost.yaml

+group:
+  - multiple_choice
+task: corypaik_prost
+dataset_path: corypaik/prost
+dataset_name: null
+output_type: multiple_choice
+test_split: test
+template_aliases: "{% set answer_choices = [A, B, C, D] %}{% set gold = label %}" # 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: "{{context}}\nQuestion:{{ex_question}}\nAnswer:"
+doc_to_target: "{{gold}}" # this will be cast to an int.
+should_decontaminate: true
+doc_to_decontamination_query: "{{context}}\nQuestion:{{ex_question}}\nAnswer:"
+metric_list:
+  - metric: acc
+    aggregation: mean
+    higher_is_better: true
+  - metric: acc_norm
+    aggregation: mean
+    higher_is_better: true
\ No newline at end of file