Enhance logging for valset usage in GEPA (#8770)

* Enhance logging for valset usage in GEPA

Clarified warning about using trainset as valset and added guidance on valset size.

* Enhance warning for missing validation set

* Refine logger messages in GEPA implementation

Updated warning and info messages for clarity and detail.

Author: LakshyAAAgrawal

dspy/teleprompt/gepa/gepa.py

@@ -428,8 +428,10 @@ def compile(
 
         logger.info(f"Running GEPA for approx {self.max_metric_calls} metric calls of the program. This amounts to {self.max_metric_calls / len(trainset) if valset is None else self.max_metric_calls / (len(trainset) + len(valset)):.2f} full evals on the {'train' if valset is None else 'train+val'} set.")
 
+        if valset is None:
+            logger.warning("No valset provided; Using trainset as valset. This is useful as an inference-time scaling strategy where you want GEPA to find the best solutions for the provided tasks in the trainse, as it make GEPA overfit prompts to the provided trainset. In order to ensure generalization and perform well on unseen tasks, please provide separate trainset and valset. Provide the smallest valset that is just large enough to match the downstream task distribution, while keeping trainset as large as possible.")
         valset = valset or trainset
-        logger.info(f"Using {len(valset)} examples for tracking Pareto scores. You can consider using a smaller sample of the valset to allow GEPA to explore more diverse solutions within the same budget.")
+        logger.info(f"Using {len(valset)} examples for tracking Pareto scores. You can consider using a smaller sample of the valset to allow GEPA to explore more diverse solutions within the same budget. GEPA requires you to provide the smallest valset that is just large enough to match your downstream task distribution, while providing as large trainset as possible.")
 
         rng = random.Random(self.seed)