add piqa generation

lm_eval/tasks/piqa/generation.yaml

+task: piqa_generation
+dataset_path: piqa
+dataset_name: null
+output_type: generate_until
+training_split: train
+validation_split: validation
+test_split: null
+doc_to_text: |-
+  Given the following goal and two candidate answers (A and B), choose the best answer
+  Goal: {{goal}}
+  A. {{sol1}}
+  B. {{sol2}}
+  Your response should end with "The best answer is [the_answer_letter]" where the [the_answer_letter] is one of A or B.
+doc_to_target: "['A', 'B'][label]"
+doc_to_decontamination_query: goal
+metric_list:
+  - metric: exact_match
+    aggregation: mean
+    higher_is_better: true
+metadata:
+  version: 1.0
+dataset_kwargs:
+  trust_remote_code: true