Fix evaluate crash on tasks with no stop sequences

[choipilkyu]

Problem

_rstrip_until in mlx_lm/evaluate.py calls min() on the list of stop-sequence positions. When a task specifies no stop sequences the list is empty and it raises:

ValueError: min() iterable argument is empty

lm-eval's ifeval does exactly that — until: [] in ifeval.yaml, because the completion is meant to run to max_gen_toks (1280) rather than stop at a marker. Every multilingual variant (ifeval_ca, ifeval_es, …) sets it too. So:

mlx_lm.evaluate --model <any> --task ifeval

fails outright instead of producing a score.

Fix

With no stop sequences there is nothing to truncate at, so the completion is returned whole. min(f, default=l) does that; the non-empty case is unchanged.

Tests

Added TestRstripUntil to tests/test_evaluate.py covering truncation at the first stop sequence, the earliest of several, no match, the empty-untils case, and empty-untils on an empty string.

$ python -m pytest tests/test_evaluate.py -q
......                                                                   [100%]
6 passed

[choipilkyu]

Reproduction against main (e5baded), using the request lm-eval actually builds for ifeval — no model needed, since the failure is in post-processing:

from lm_eval.tasks import TaskManager, get_task_dict
from mlx_lm.evaluate import _rstrip_until

task = get_task_dict(["ifeval"], TaskManager())["ifeval"]
task.build_all_requests(limit=1, rank=0, world_size=1)
ctx, opt = task.instances[0].args
print(opt)
_rstrip_until("some completion", opt["until"])

{'until': [], 'do_sample': False, 'temperature': 0.0, 'max_gen_toks': 1280}
Traceback (most recent call last):
  File "mlx_lm/evaluate.py", line 38, in _rstrip_until
    return s[: min(f)]
               ~~~^^^
ValueError: min() iterable argument is empty

generate_until passes opt["until"] straight through at evaluate.py:360 with no normalisation, so the empty list from the task config reaches _rstrip_until unchanged.

lm-eval 0.4.12, lm_eval/tasks/ifeval/ifeval.yaml.