Add support for generative answering of multiple_choice tasks

[pasky]

As they are, multiple_choice tasks cannot be evaluated with popular API models that support only generate_until, not logprob.

This MR introduces a flag that allows these tasks to be still evaluated by introducing an emulation mode of sorts that just asks the model to generate the answer. The abcd approach and most of the prompt is borrowed from openai/evals.

[pasky]

For reference, some tinyBenchmark results with claude:

lm_eval  --tasks tinyArc --model local-chat-completions --model_args model=claude-3-5-sonnet-20241022,base_url=http://localhost:4000/v1/chat/completions,num_concurrent=4,max_retries=3,tokenized_requests=False --gen_kwargs until='qwelrjh',max_tokens=512 --log_samples --output_path results0 --apply_chat_template --fewshot_as_multiturn  --multiple_choice_generate abcd --hf_hub_log_args hub_repo_name=eval-claude-3-5-sonnet-20241022,push_results_to_hub,push_samples_to_hub,public_repo
local-chat-completions (model=claude-3-5-sonnet-20241022,base_url=http://localhost:4000/v1/chat/completions,num_concurrent=4,max_retries=3,tokenized_requests=False), gen_kwargs: (until=qwelrjh,max_tokens=512), limit: None, num_fewshot: None, batch_size: 1
| Tasks |Version|Filter|n-shot| Metric |   |Value |   |Stderr|
|-------|------:|------|-----:|--------|---|-----:|---|------|
|tinyArc|      0|none  |    25|acc_norm|↑  |0.8819|±  |   N/A|

lm_eval  --tasks tinyHellaswag --model local-chat-completions --model_args model=claude-3-5-sonnet-20241022,base_url=http://localhost:4000/v1/chat/completions,num_concurrent=4,max_retries=3,tokenized_requests=False --gen_kwargs until='qwelrjh',max_tokens=512 --log_samples --output_path results0 --apply_chat_template --fewshot_as_multiturn  --multiple_choice_generate abcd --hf_hub_log_args hub_repo_name=eval-claude-3-5-sonnet-20241022,push_results_to_hub,push_samples_to_hub,public_repo
local-chat-completions (model=claude-3-5-sonnet-20241022,base_url=http://localhost:4000/v1/chat/completions,num_concurrent=4,max_retries=3,tokenized_requests=False), gen_kwargs: (until=qwelrjh,max_tokens=512), limit: None, num_fewshot: None, batch_size: 1
|    Tasks    |Version|Filter|n-shot| Metric |   |Value |   |Stderr|
|-------------|------:|------|-----:|--------|---|-----:|---|------|
|tinyHellaswag|      0|none  |    10|acc_norm|↑  |0.8283|±  |   N/A|

lm_eval  --tasks tinyMMLU --model local-chat-completions --model_args model=claude-3-5-sonnet-20241022,base_url=http://localhost:4000/v1/chat/completions,num_concurrent=4,max_retries=3,tokenized_requests=False --gen_kwargs until='qwelrjh',max_tokens=512 --log_samples --output_path results0 --apply_chat_template --fewshot_as_multiturn  --multiple_choice_generate abcd --hf_hub_log_args hub_repo_name=eval-claude-3-5-sonnet-20241022,push_results_to_hub,push_samples_to_hub,public_repo
local-chat-completions (model=claude-3-5-sonnet-20241022,base_url=http://localhost:4000/v1/chat/completions,num_concurrent=4,max_retries=3,tokenized_requests=False), gen_kwargs: (until=qwelrjh,max_tokens=512), limit: None, num_fewshot: None, batch_size: 1
| Tasks  |Version|Filter|n-shot| Metric |   |Value|   |Stderr|
|--------|------:|------|-----:|--------|---|----:|---|------|
|tinyMMLU|      0|none  |     0|acc_norm|↑  |0.789|±  |   N/A|

[LSinev]

May I suggest to not hardcode these. What if doc_system_instruction supposed to be delimited with some other delimiter? What if set of choices is not 4 letters, not these 4 letters, or not letters at all? This framework supports external tasks and also have multiple forks already, so there may be (I am not using "are" because of no intention to google proof of this idea) multiple choice tasks set up differently than "abcd".

[LSinev]

What about non-english tasks that are already inside this repo?