Data collator for token classification (#8274)

* Add DataCollatorForTokenClassification and clean tests

* Make quality

src/transformers/data/data_collator.py

@@ -114,6 +114,67 @@ class DataCollatorWithPadding:
         return batch
 
 
+@dataclass
+class DataCollatorForTokenClassification:
+    """
+    Data collator that will dynamically pad the inputs received, as well as the labels.
+
+    Args:
+        tokenizer (:class:`~transformers.PreTrainedTokenizer` or :class:`~transformers.PreTrainedTokenizerFast`):
+            The tokenizer used for encoding the data.
+        padding (:obj:`bool`, :obj:`str` or :class:`~transformers.tokenization_utils_base.PaddingStrategy`, `optional`, defaults to :obj:`True`):
+            Select a strategy to pad the returned sequences (according to the model's padding side and padding index)
+            among:
+
+            * :obj:`True` or :obj:`'longest'`: Pad to the longest sequence in the batch (or no padding if only a single
+              sequence if provided).
+            * :obj:`'max_length'`: Pad to a maximum length specified with the argument :obj:`max_length` or to the
+              maximum acceptable input length for the model if that argument is not provided.
+            * :obj:`False` or :obj:`'do_not_pad'` (default): No padding (i.e., can output a batch with sequences of
+              different lengths).
+        max_length (:obj:`int`, `optional`):
+            Maximum length of the returned list and optionally padding length (see above).
+        pad_to_multiple_of (:obj:`int`, `optional`):
+            If set will pad the sequence to a multiple of the provided value.
+
+            This is especially useful to enable the use of Tensor Cores on NVIDIA hardware with compute capability >=
+            7.5 (Volta).
+        label_pad_token_id (:obj:`int`, `optional`, defaults to -100):
+            The id to use when padding the labels (-100 will be automatically ignore by PyTorch loss functions).
+    """
+
+    tokenizer: PreTrainedTokenizerBase
+    padding: Union[bool, str, PaddingStrategy] = True
+    max_length: Optional[int] = None
+    pad_to_multiple_of: Optional[int] = None
+    label_pad_token_id: int = -100
+
+    def __call__(self, features):
+        label_name = "label" if "label" in features[0].keys() else "labels"
+        labels = [feature[label_name] for feature in features] if label_name in features[0].keys() else None
+        batch = self.tokenizer.pad(
+            features,
+            padding=self.padding,
+            max_length=self.max_length,
+            pad_to_multiple_of=self.pad_to_multiple_of,
+            # Conversion to tensors will fail if we have labels as they are not of the same length yet.
+            return_tensors="pt" if labels is None else None,
+        )
+
+        if labels is None:
+            return batch
+
+        sequence_length = torch.tensor(batch["input_ids"]).shape[1]
+        padding_side = self.tokenizer.padding_side
+        if padding_side == "right":
+            batch["labels"] = [label + [self.label_pad_token_id] * (sequence_length - len(label)) for label in labels]
+        else:
+            batch["labels"] = [[self.label_pad_token_id] * (sequence_length - len(label)) + label for label in labels]
+
+        batch = {k: torch.tensor(v, dtype=torch.int64) for k, v in batch.items()}
+        return batch
+
+
 @dataclass
 class DataCollatorForLanguageModeling:
     """