fix LTO for low tissue validations sets

drevalpy/datasets/dataset.py

@@ -551,17 +551,34 @@ def _split_early_stopping_data(
 
     :param validation_dataset: input validation dataset
     :param test_mode: LPO, LCO, LTO, LDO
+    :raises ValueError: if test_mode is not one of the expected values
     :returns: the resulting validation and early stopping datasets
     """
     validation_dataset.shuffle(random_state=42)
+
+    # Determine the number of splits b (default 4,
+    # but can be less if there are not enough groups)
+    if test_mode == "LTO":
+        tissues = validation_dataset.tissue
+        if tissues is None:
+            raise ValueError("Tissue information is required for LTO.")
+        n_splits = min(4, len(np.unique(tissues)))
+    elif test_mode == "LCO":
+        n_splits = min(4, len(np.unique(validation_dataset.cell_line_ids)))
+    elif test_mode == "LDO":
+        n_splits = min(4, len(np.unique(validation_dataset.drug_ids)))
+    else:
+        n_splits = 4
+
     cv_v = validation_dataset.split_dataset(
-        n_cv_splits=4,
+        n_cv_splits=n_splits,
         mode=test_mode,
         split_validation=False,
         split_early_stopping=False,
         random_state=42,
     )
     # take the first fold of a 4 cv as the split i.e. 3/4 for validation and 1/4 for early stopping
+    # when n_groups is less than 4, we splits the validation dataset into 2/3 and 1/3 or 1/2 and 1/2
     validation_dataset = cv_v[0]["train"]
     early_stopping_dataset = cv_v[0]["test"]
     return validation_dataset, early_stopping_dataset