Fix RecursionError in imdb_contrastset_prompt

[pjavanrood]

Describe the bug

The imdb_contrastset_prompt function contained an incorrect call to the task config.

To Reproduce

task = "imdb:contrastset|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()

TypeError                                 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 = "imdb:contrastset|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()
...
     43         choices=["Positive", "Negative"],
     44         gold_index=["Positive", "Negative"].index(line["contrast_references"]),
     45     )

TypeError: 'LightevalTaskConfig' object is not callable

Expected behavior

The function should correctly delegate to the base imdb_prompt and return a valid Doc object.

Version info

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