Fix split loading error in bigbench

[pjavanrood]

Describe the bug

The bigbench tasks failed with ValueError or KeyError because the evaluation_splits were set to ["validation"] or ["test"] improperly for some subsets which only had a default or train split.

To Reproduce

from datasets import load_dataset
task = "bigbench:mult_data_wrangling|5"

pipeline = Pipeline(
    tasks=task,
    pipeline_parameters=pipeline_params,
    evaluation_tracker=evaluation_tracker,
    model_config=model_config,
)

pipeline.evaluate()
pipeline.save_and_push_results()
pipeline.show_results()

---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[3], line 29
     18 # model_config = LiteLLMModelConfig(
     19 #     model_name="openai/gpt-4.1",
     20 #     base_url="https://api.openai.com/v1",
   (...)     24 #     )
     25 # )
     27 task = "bigbench:mult_data_wrangling|5"
---> 29 pipeline = Pipeline(
     30     tasks=task,
     31     pipeline_parameters=pipeline_params,
     32     evaluation_tracker=evaluation_tracker,
     33     model_config=model_config,
     34 )
     36 pipeline.evaluate()
     37 pipeline.save_and_push_results()

File ~/Code/Evalhub/backend/lighteval/src/lighteval/pipeline.py:142, in Pipeline.__init__(self, tasks, pipeline_parameters, evaluation_tracker, model_config, model, metric_options)
    140 # We init tasks first to fail fast if one is badly defined
    141 self._init_random_seeds()
--> 142 self._init_tasks_and_requests(tasks=tasks)
    144 self.model_config = model_config
    145 self.accelerator, self.parallel_context = self._init_parallelism_manager()
...
     88         available_suggested_splits = [
     89             split for split in (Split.TRAIN, Split.TEST, Split.VALIDATION) if split in self
     90         ]

KeyError: 'default'

Expected behavior

The configuration should point to existing splits in the Hugging Face dataset.

Version info

• OS: mac
• Lighteval version: main (local development)