Make default_data_collator more flexible and deprecate old behavior (#5060)

* Make default_data_collator more flexible

* Accept tensors for all features

* Document code

* Refactor

* Formatting

src/transformers/data/data_collator.py

@@ -33,31 +33,34 @@ def default_data_collator(features: List[InputDataClass]) -> Dict[str, torch.Ten
     # have the same attributes.
     # So we will look at the first element as a proxy for what attributes exist
     # on the whole batch.
+    if not isinstance(features[0], dict):
+        features = [vars(f) for f in features]
+
     first = features[0]
+    batch = {}
 
     # Special handling for labels.
     # Ensure that tensor is created with the correct type
     # (it should be automatically the case, but let's make sure of it.)
-    if hasattr(first, "label") and first.label is not None:
-        if type(first.label) is int:
-            labels = torch.tensor([f.label for f in features], dtype=torch.long)
-        else:
-            labels = torch.tensor([f.label for f in features], dtype=torch.float)
-        batch = {"labels": labels}
-    elif hasattr(first, "label_ids") and first.label_ids is not None:
-        if type(first.label_ids[0]) is int:
-            labels = torch.tensor([f.label_ids for f in features], dtype=torch.long)
+    if "label" in first:
+        dtype = torch.long if type(first["label"]) is int else torch.float
+        batch["labels"] = torch.tensor([f["label"] for f in features], dtype=dtype)
+    elif "label_ids" in first:
+        if isinstance(first["label_ids"], torch.Tensor):
+            batch["labels"] = torch.stack([f["label_ids"] for f in features])
         else:
-            labels = torch.tensor([f.label_ids for f in features], dtype=torch.float)
-        batch = {"labels": labels}
-    else:
-        batch = {}
+            dtype = torch.long if type(first["label_ids"][0]) is int else torch.float
+            batch["labels"] = torch.tensor([f["label_ids"] for f in features], dtype=dtype)
 
-    # Handling of all other possible attributes.
+    # Handling of all other possible keys.
     # Again, we will use the first element to figure out which key/values are not None for this model.
-    for k, v in vars(first).items():
+    for k, v in first.items():
         if k not in ("label", "label_ids") and v is not None and not isinstance(v, str):
-            batch[k] = torch.tensor([getattr(f, k) for f in features], dtype=torch.long)
+            if isinstance(v, torch.Tensor):
+                batch[k] = torch.stack([f[k] for f in features])
+            else:
+                batch[k] = torch.tensor([f[k] for f in features], dtype=torch.long)
+
     return batch
 
 

src/transformers/trainer.py

@@ -4,6 +4,7 @@ import os
 import random
 import re
 import shutil
+import warnings
 from contextlib import contextmanager
 from pathlib import Path
 from typing import Callable, Dict, List, Optional, Tuple
@@ -205,6 +206,15 @@ class Trainer:
             # Set an xla_device flag on the model's config.
             # We'll find a more elegant and not need to do this in the future.
             self.model.config.xla_device = True
+        if not callable(self.data_collator) and callable(getattr(self.data_collator, "collate_batch", None)):
+            self.data_collator = self.data_collator.collate_batch
+            warnings.warn(
+                (
+                    "The `data_collator` should now be a simple callable (function, class with `__call__`), classes "
+                    + "with a `collate_batch` are deprecated and won't be supported in a future version."
+                ),
+                FutureWarning,
+            )
 
     def get_train_dataloader(self) -> DataLoader:
         if self.train_dataset is None:

tests/test_trainer.py

@@ -24,6 +24,27 @@ PATH_SAMPLE_TEXT = "./tests/fixtures/sample_text.txt"
 
 @require_torch
 class DataCollatorIntegrationTest(unittest.TestCase):
+    def test_default_with_dict(self):
+        features = [{"labels": i, "inputs": [0, 1, 2, 3, 4, 5]} for i in range(8)]
+        batch = default_data_collator(features)
+        self.assertTrue(batch["labels"].equal(torch.tensor(list(range(8)))))
+        self.assertEqual(batch["labels"].dtype, torch.long)
+        self.assertEqual(batch["inputs"].shape, torch.Size([8, 6]))
+
+        # With label_ids
+        features = [{"label_ids": [0, 1, 2], "inputs": [0, 1, 2, 3, 4, 5]} for i in range(8)]
+        batch = default_data_collator(features)
+        self.assertTrue(batch["labels"].equal(torch.tensor([[0, 1, 2]] * 8)))
+        self.assertEqual(batch["labels"].dtype, torch.long)
+        self.assertEqual(batch["inputs"].shape, torch.Size([8, 6]))
+
+        # Features can already be tensors
+        features = [{"labels": i, "inputs": torch.randint(10, [10])} for i in range(8)]
+        batch = default_data_collator(features)
+        self.assertTrue(batch["labels"].equal(torch.tensor(list(range(8)))))
+        self.assertEqual(batch["labels"].dtype, torch.long)
+        self.assertEqual(batch["inputs"].shape, torch.Size([8, 10]))
+
     def test_default_classification(self):
         MODEL_ID = "bert-base-cased-finetuned-mrpc"
         tokenizer = AutoTokenizer.from_pretrained(MODEL_ID)