Fix CLI --batch_size arg for openai-completions/local-completions (EleutherAI#1656)

The OpenAI interface supports batch size as an argument to the completions API, but does not seem to support specification of this on the CLI i.e. `lm_eval --model openai-completions --batch_size 16 ...` because of a simple lack of str->int conversion.

This is confirmed by my usage and stacktrace from running `OPENAI_API_KEY=dummy lm_eval --model local-completions --tasks gsm8k --batch_size 16 --model_args model=nm-
testing/zephyr-beta-7b-gptq-g128,tokenizer_backend=huggingface,base_url=http://localhost:8000/v1`:
```
Traceback (most recent call last):
  File "/home/michael/venv/bin/lm_eval", line 8, in <module>
    sys.exit(cli_evaluate())
  File "/home/michael/code/lm-evaluation-harness/lm_eval/__main__.py", line 341, in cli_evaluate
    results = evaluator.simple_evaluate(
  File "/home/michael/code/lm-evaluation-harness/lm_eval/utils.py", line 288, in _wrapper
    return fn(*args, **kwargs)
  File "/home/michael/code/lm-evaluation-harness/lm_eval/evaluator.py", line 251, in simple_evaluate
    results = evaluate(
  File "/home/michael/code/lm-evaluation-harness/lm_eval/utils.py", line 288, in _wrapper
    return fn(*args, **kwargs)
  File "/home/michael/code/lm-evaluation-harness/lm_eval/evaluator.py", line 390, in evaluate
    resps = getattr(lm, reqtype)(cloned_reqs)
  File "/home/michael/code/lm-evaluation-harness/lm_eval/models/openai_completions.py", line 263, in generate_until
    list(sameuntil_chunks(re_ord.get_reordered(), self.batch_size)),
  File "/home/michael/code/lm-evaluation-harness/lm_eval/models/openai_completions.py", line 251, in sameuntil_chunks
    if len(ret) >= size or x[1] != lastuntil:
TypeError: '>=' not supported between instances of 'int' and 'str'
```

Author: mgoin

lm_eval/models/openai_completions.py

@@ -111,7 +111,7 @@ def __init__(
         self.base_url = base_url
         self.tokenizer_backend = tokenizer_backend
         self.truncate = truncate
-        self._batch_size = batch_size
+        self._batch_size = int(batch_size)
         self._max_gen_toks = max_gen_toks
         self._max_length = max_length