add missing crossval_splits.py file

nnunetv2/utilities/crossval_split.py

@@ -0,0 +1,16 @@
+from typing import List
+
+import numpy as np
+from sklearn.model_selection import KFold
+
+
+def generate_crossval_split(train_identifiers: List[str], seed=12345, n_splits=5) -> List[dict[str, List[str]]]:
+    splits = []
+    kfold = KFold(n_splits=n_splits, shuffle=True, random_state=seed)
+    for i, (train_idx, test_idx) in enumerate(kfold.split(train_identifiers)):
+        train_keys = np.array(train_identifiers)[train_idx]
+        test_keys = np.array(train_identifiers)[test_idx]
+        splits.append({})
+        splits[-1]['train'] = list(train_keys)
+        splits[-1]['val'] = list(test_keys)
+    return splits