LayoutLMv2Processor: ensure 1-to-1 mapping between images and samples in case of overflowing tokens (#17092)

* add get_overflowing_images function to ensure 1-to-1 mapping between samples and images in LayoutLMv2Processor

* make style

* add test for overflowing_tokens, change assert to ValueError, avoiding unrelated formatting changes

* change line length by passing --preview into black

src/transformers/models/layoutlmv2/processing_layoutlmv2.py

@@ -86,10 +86,12 @@ class LayoutLMv2Processor(ProcessorMixin):
 
         if self.feature_extractor.apply_ocr and (word_labels is not None):
             raise ValueError(
-                "You cannot provide word labels "
-                "if you initialized the feature extractor with apply_ocr set to True."
+                "You cannot provide word labels if you initialized the feature extractor with apply_ocr set to True."
             )
 
+        if return_overflowing_tokens is True and return_offsets_mapping is False:
+            raise ValueError("You cannot return overflowing tokens without returning the offsets mapping.")
+
         # first, apply the feature extractor
         features = self.feature_extractor(images=images, return_tensors=return_tensors)
 
@@ -122,6 +124,23 @@ class LayoutLMv2Processor(ProcessorMixin):
         )
 
         # add pixel values
-        encoded_inputs["image"] = features.pop("pixel_values")
+        images = features.pop("pixel_values")
+        if return_overflowing_tokens is True:
+            images = self.get_overflowing_images(images, encoded_inputs["overflow_to_sample_mapping"])
+        encoded_inputs["image"] = images
 
         return encoded_inputs
+
+    def get_overflowing_images(self, images, overflow_to_sample_mapping):
+        # in case there's an overflow, ensure each `input_ids` sample is mapped to its corresponding image
+        images_with_overflow = []
+        for sample_idx in overflow_to_sample_mapping:
+            images_with_overflow.append(images[sample_idx])
+
+        if len(images_with_overflow) != len(overflow_to_sample_mapping):
+            raise ValueError(
+                "Expected length of images to be the same as the length of `overflow_to_sample_mapping`, but got"
+                f" {len(images_with_overflow)} and {len(overflow_to_sample_mapping)}"
+            )
+
+        return images_with_overflow

tests/models/layoutlmv2/test_processor_layoutlmv2.py

@@ -133,6 +133,39 @@ class LayoutLMv2ProcessorTest(unittest.TestCase):
         self.assertEqual(processor.feature_extractor.to_json_string(), feature_extractor_add_kwargs.to_json_string())
         self.assertIsInstance(processor.feature_extractor, LayoutLMv2FeatureExtractor)
 
+    @slow
+    def test_overflowing_tokens(self):
+        # In the case of overflowing tokens, test that we still have 1-to-1 mapping between the images and input_ids (sequences that are too long are broken down into multiple sequences).
+
+        from datasets import load_dataset
+
+        # set up
+        datasets = load_dataset("nielsr/funsd")
+        processor = LayoutLMv2Processor.from_pretrained("microsoft/layoutlmv2-base-uncased", revision="no_ocr")
+
+        def preprocess_data(examples):
+            images = [Image.open(path).convert("RGB") for path in examples["image_path"]]
+            words = examples["words"]
+            boxes = examples["bboxes"]
+            word_labels = examples["ner_tags"]
+            encoded_inputs = processor(
+                images,
+                words,
+                boxes=boxes,
+                word_labels=word_labels,
+                padding="max_length",
+                truncation=True,
+                return_overflowing_tokens=True,
+                stride=50,
+                return_offsets_mapping=True,
+                return_tensors="pt",
+            )
+            return encoded_inputs
+
+        train_data = preprocess_data(datasets["train"])
+
+        self.assertEqual(len(train_data["image"]), len(train_data["input_ids"]))
+
 
 # different use cases tests
 @require_torch