Merge pull request #627 from fattorib/refactor-more-tasks

[Refactor] Add: SWAG,RACE,Arithmetic,Winogrande,PubmedQA

lm_eval/tasks/winogrande/winogrande.yaml

+task: winogrande
+dataset_path: winogrande
+dataset_name: winogrande_xl
+output_type: winograd_schema
+training_split: train
+validation_split: validation
+doc_to_target: !function preprocess_winogrande.partial_target
+doc_to_text: "{{sentence}}"
+create_choices: !function preprocess_winogrande.create_choices
+gold_alias: !function preprocess_winogrande.gold_alias
+metric_list:
+  - metric: acc
+    aggregation: mean
+    higher_is_better: true