Fix infer list of images (#7667)

fix infer list of images

Author: lhoestq

src/datasets/arrow_writer.py

@@ -28,6 +28,7 @@
 from .features import Audio, Features, Image, Pdf, Value, Video
 from .features.features import (
     FeatureType,
+    List,
     _ArrayXDExtensionType,
     _visit,
     cast_to_python_objects,
@@ -193,19 +194,19 @@ def _infer_custom_type_and_encode(data: Iterable) -> tuple[Iterable, Optional[Fe
             if isinstance(non_null_value, PIL.Image.Image):
                 return [Image().encode_example(value) if value is not None else None for value in data], Image()
             if isinstance(non_null_value, list) and isinstance(non_null_value[0], PIL.Image.Image):
-                return [[Image().encode_example(x) for x in value] if value is not None else None for value in data], [
-                    Image()
-                ]
+                return [
+                    [Image().encode_example(x) for x in value] if value is not None else None for value in data
+                ], List(Image())
         if config.PDFPLUMBER_AVAILABLE and "pdfplumber" in sys.modules:
             import pdfplumber
 
             non_null_idx, non_null_value = first_non_null_non_empty_value(data)
             if isinstance(non_null_value, pdfplumber.pdf.PDF):
                 return [Pdf().encode_example(value) if value is not None else None for value in data], Pdf()
             if isinstance(non_null_value, list) and isinstance(non_null_value[0], pdfplumber.pdf.PDF):
-                return [[Pdf().encode_example(x) for x in value] if value is not None else None for value in data], [
-                    Pdf()
-                ]
+                return [
+                    [Pdf().encode_example(x) for x in value] if value is not None else None for value in data
+                ], List(Pdf())
         return data, None
 
     def __arrow_array__(self, type: Optional[pa.DataType] = None):