Longbench v2 (#3338)

* initial commit

* change to acc

* fix long-dialogue tasks

* fix versioning

* more fixes

* fix naming

* fix naming

* more renaming

* maybe a dataset fix

* fix dataset and use new dataset schema

* add README

* fix prompt and dataset naming

* lint

* remove utils.py

* lint

* more linting

* fix typo

* fix naming

* add longbenchv2

---------
Co-authored-by: Baber <baber@hey.com>

lm_eval/tasks/longbench2/legal_multi.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_multi
+task: longbench2_legal_multi
+dataset_name: legal_multi

lm_eval/tasks/longbench2/legal_single.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_single
+task: longbench2_legal_single
+dataset_name: legal_single

lm_eval/tasks/longbench2/lit_single_doc.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_single
+task: longbench2_lit_single
+dataset_name: literary

lm_eval/tasks/longbench2/longbench2_code.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+task: longbench2_code
+dataset_name: code_repo_qa

lm_eval/tasks/longbench2/many_shot.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_incontext
+task: longbench2_many_shot
+dataset_name: manyshot_learning

lm_eval/tasks/longbench2/news_multi.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_multi
+task: longbench2_news_multi
+dataset_name: multinews

lm_eval/tasks/longbench2/table.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_structured
+task: longbench2_table
+dataset_name: table_qa

lm_eval/tasks/longbench2/translate.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_incontext
+task: longbench2_translate
+dataset_name: new_language_translation

lm_eval/tasks/longbench2/user_guide.yaml

+include: _longbench_common_yaml
+tag:
+  - longbench2
+  - longbench2_incontext
+task: longbench2_user_guide
+dataset_name: user_guide_qa