Remove roberta dependency from longformer fast tokenizer (#19501)

* remove roberta fast tokenizer dependency

* fix flake8

* Update src/transformers/models/longformer/tokenization_longformer_fast.py
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/models/longformer/tokenization_longformer_fast.py

@@ -12,15 +12,20 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+"""Fast Tokenization classes for Longformer."""
+import json
+from typing import List, Optional, Tuple
 
+from tokenizers import pre_tokenizers, processors
+
+from ...tokenization_utils_base import AddedToken, BatchEncoding
+from ...tokenization_utils_fast import PreTrainedTokenizerFast
 from ...utils import logging
-from ..roberta.tokenization_roberta_fast import RobertaTokenizerFast
 from .tokenization_longformer import LongformerTokenizer
 
 
 logger = logging.get_logger(__name__)
 
-
 VOCAB_FILES_NAMES = {"vocab_file": "vocab.json", "merges_file": "merges.txt", "tokenizer_file": "tokenizer.json"}
 
 PRETRAINED_VOCAB_FILES_MAP = {
@@ -82,15 +87,236 @@ PRETRAINED_POSITIONAL_EMBEDDINGS_SIZES = {
 }
 
 
-class LongformerTokenizerFast(RobertaTokenizerFast):
-    r"""
-    Construct a "fast" Longformer tokenizer (backed by HuggingFace's *tokenizers* library).
+# Copied from transformers.models.roberta.tokenization_roberta_fast.RobertaTokenizerFast with roberta-base->allenai/longformer-base-4096, RoBERTa->Longformer all-casing, Roberta->Longformer
+class LongformerTokenizerFast(PreTrainedTokenizerFast):
+    """
+    Construct a "fast" Longformer tokenizer (backed by HuggingFace's *tokenizers* library), derived from the GPT-2
+    tokenizer, using byte-level Byte-Pair-Encoding.
+
+    This tokenizer has been trained to treat spaces like parts of the tokens (a bit like sentencepiece) so a word will
+    be encoded differently whether it is at the beginning of the sentence (without space) or not:
+
+    ```
+    >>> from transformers import LongformerTokenizerFast
+    >>> tokenizer = LongformerTokenizerFast.from_pretrained("allenai/longformer-base-4096")
+    >>> tokenizer("Hello world")['input_ids']
+    [0, 31414, 232, 328, 2]
+    >>> tokenizer(" Hello world")['input_ids']
+    [0, 20920, 232, 2]
+    ```
+
+    You can get around that behavior by passing `add_prefix_space=True` when instantiating this tokenizer or when you
+    call it on some text, but since the model was not pretrained this way, it might yield a decrease in performance.
+
+    <Tip>
+
+    When used with `is_split_into_words=True`, this tokenizer needs to be instantiated with `add_prefix_space=True`.
+
+    </Tip>
+
+    This tokenizer inherits from [`PreTrainedTokenizerFast`] which contains most of the main methods. Users should
+    refer to this superclass for more information regarding those methods.
+
+    Args:
+        vocab_file (`str`):
+            Path to the vocabulary file.
+        merges_file (`str`):
+            Path to the merges file.
+        errors (`str`, *optional*, defaults to `"replace"`):
+            Paradigm to follow when decoding bytes to UTF-8. See
+            [bytes.decode](https://docs.python.org/3/library/stdtypes.html#bytes.decode) for more information.
+        bos_token (`str`, *optional*, defaults to `"<s>"`):
+            The beginning of sequence token that was used during pretraining. Can be used a sequence classifier token.
 
-    [`LongformerTokenizerFast`] is identical to [`RobertaTokenizerFast`]. Refer to the superclass for usage examples
-    and documentation concerning parameters.
+            <Tip>
+
+            When building a sequence using special tokens, this is not the token that is used for the beginning of
+            sequence. The token used is the `cls_token`.
+
+            </Tip>
+
+        eos_token (`str`, *optional*, defaults to `"</s>"`):
+            The end of sequence token.
+
+            <Tip>
+
+            When building a sequence using special tokens, this is not the token that is used for the end of sequence.
+            The token used is the `sep_token`.
+
+            </Tip>
+
+        sep_token (`str`, *optional*, defaults to `"</s>"`):
+            The separator token, which is used when building a sequence from multiple sequences, e.g. two sequences for
+            sequence classification or for a text and a question for question answering. It is also used as the last
+            token of a sequence built with special tokens.
+        cls_token (`str`, *optional*, defaults to `"<s>"`):
+            The classifier token which is used when doing sequence classification (classification of the whole sequence
+            instead of per-token classification). It is the first token of the sequence when built with special tokens.
+        unk_token (`str`, *optional*, defaults to `"<unk>"`):
+            The unknown token. A token that is not in the vocabulary cannot be converted to an ID and is set to be this
+            token instead.
+        pad_token (`str`, *optional*, defaults to `"<pad>"`):
+            The token used for padding, for example when batching sequences of different lengths.
+        mask_token (`str`, *optional*, defaults to `"<mask>"`):
+            The token used for masking values. This is the token used when training this model with masked language
+            modeling. This is the token which the model will try to predict.
+        add_prefix_space (`bool`, *optional*, defaults to `False`):
+            Whether or not to add an initial space to the input. This allows to treat the leading word just as any
+            other word. (Longformer tokenizer detect beginning of words by the preceding space).
+        trim_offsets (`bool`, *optional*, defaults to `True`):
+            Whether the post processing step should trim offsets to avoid including whitespaces.
     """
-    # merges and vocab same as Roberta
+
     vocab_files_names = VOCAB_FILES_NAMES
     pretrained_vocab_files_map = PRETRAINED_VOCAB_FILES_MAP
     max_model_input_sizes = PRETRAINED_POSITIONAL_EMBEDDINGS_SIZES
+    model_input_names = ["input_ids", "attention_mask"]
     slow_tokenizer_class = LongformerTokenizer
+
+    def __init__(
+        self,
+        vocab_file=None,
+        merges_file=None,
+        tokenizer_file=None,
+        errors="replace",
+        bos_token="<s>",
+        eos_token="</s>",
+        sep_token="</s>",
+        cls_token="<s>",
+        unk_token="<unk>",
+        pad_token="<pad>",
+        mask_token="<mask>",
+        add_prefix_space=False,
+        trim_offsets=True,
+        **kwargs
+    ):
+        super().__init__(
+            vocab_file,
+            merges_file,
+            tokenizer_file=tokenizer_file,
+            errors=errors,
+            bos_token=bos_token,
+            eos_token=eos_token,
+            sep_token=sep_token,
+            cls_token=cls_token,
+            unk_token=unk_token,
+            pad_token=pad_token,
+            mask_token=mask_token,
+            add_prefix_space=add_prefix_space,
+            trim_offsets=trim_offsets,
+            **kwargs,
+        )
+
+        pre_tok_state = json.loads(self.backend_tokenizer.pre_tokenizer.__getstate__())
+        if pre_tok_state.get("add_prefix_space", add_prefix_space) != add_prefix_space:
+            pre_tok_class = getattr(pre_tokenizers, pre_tok_state.pop("type"))
+            pre_tok_state["add_prefix_space"] = add_prefix_space
+            self.backend_tokenizer.pre_tokenizer = pre_tok_class(**pre_tok_state)
+
+        self.add_prefix_space = add_prefix_space
+
+        tokenizer_component = "post_processor"
+        tokenizer_component_instance = getattr(self.backend_tokenizer, tokenizer_component, None)
+        if tokenizer_component_instance:
+            state = json.loads(tokenizer_component_instance.__getstate__())
+
+            # The lists 'sep' and 'cls' must be cased in tuples for the object `post_processor_class`
+            if "sep" in state:
+                state["sep"] = tuple(state["sep"])
+            if "cls" in state:
+                state["cls"] = tuple(state["cls"])
+
+            changes_to_apply = False
+
+            if state.get("add_prefix_space", add_prefix_space) != add_prefix_space:
+                state["add_prefix_space"] = add_prefix_space
+                changes_to_apply = True
+
+            if state.get("trim_offsets", trim_offsets) != trim_offsets:
+                state["trim_offsets"] = trim_offsets
+                changes_to_apply = True
+
+            if changes_to_apply:
+                component_class = getattr(processors, state.pop("type"))
+                new_value = component_class(**state)
+                setattr(self.backend_tokenizer, tokenizer_component, new_value)
+
+    @property
+    def mask_token(self) -> str:
+        """
+        `str`: Mask token, to use when training a model with masked-language modeling. Log an error if used while not
+        having been set.
+
+        Longformer tokenizer has a special mask token to be usable in the fill-mask pipeline. The mask token will
+        greedily comprise the space before the *<mask>*.
+        """
+        if self._mask_token is None:
+            if self.verbose:
+                logger.error("Using mask_token, but it is not set yet.")
+            return None
+        return str(self._mask_token)
+
+    @mask_token.setter
+    def mask_token(self, value):
+        """
+        Overriding the default behavior of the mask token to have it eat the space before it.
+
+        This is needed to preserve backward compatibility with all the previously used models based on Longformer.
+        """
+        # Mask token behave like a normal word, i.e. include the space before it
+        # So we set lstrip to True
+        value = AddedToken(value, lstrip=True, rstrip=False) if isinstance(value, str) else value
+        self._mask_token = value
+
+    def _batch_encode_plus(self, *args, **kwargs) -> BatchEncoding:
+        is_split_into_words = kwargs.get("is_split_into_words", False)
+        assert self.add_prefix_space or not is_split_into_words, (
+            f"You need to instantiate {self.__class__.__name__} with add_prefix_space=True "
+            "to use it with pretokenized inputs."
+        )
+
+        return super()._batch_encode_plus(*args, **kwargs)
+
+    def _encode_plus(self, *args, **kwargs) -> BatchEncoding:
+        is_split_into_words = kwargs.get("is_split_into_words", False)
+
+        assert self.add_prefix_space or not is_split_into_words, (
+            f"You need to instantiate {self.__class__.__name__} with add_prefix_space=True "
+            "to use it with pretokenized inputs."
+        )
+
+        return super()._encode_plus(*args, **kwargs)
+
+    def save_vocabulary(self, save_directory: str, filename_prefix: Optional[str] = None) -> Tuple[str]:
+        files = self._tokenizer.model.save(save_directory, name=filename_prefix)
+        return tuple(files)
+
+    def build_inputs_with_special_tokens(self, token_ids_0, token_ids_1=None):
+        output = [self.bos_token_id] + token_ids_0 + [self.eos_token_id]
+        if token_ids_1 is None:
+            return output
+
+        return output + [self.eos_token_id] + token_ids_1 + [self.eos_token_id]
+
+    def create_token_type_ids_from_sequences(
+        self, token_ids_0: List[int], token_ids_1: Optional[List[int]] = None
+    ) -> List[int]:
+        """
+        Create a mask from the two sequences passed to be used in a sequence-pair classification task. Longformer does
+        not make use of token type ids, therefore a list of zeros is returned.
+
+        Args:
+            token_ids_0 (`List[int]`):
+                List of IDs.
+            token_ids_1 (`List[int]`, *optional*):
+                Optional second list of IDs for sequence pairs.
+
+        Returns:
+            `List[int]`: List of zeros.
+        """
+        sep = [self.sep_token_id]
+        cls = [self.cls_token_id]
+
+        if token_ids_1 is None:
+            return len(cls + token_ids_0 + sep) * [0]
+        return len(cls + token_ids_0 + sep + sep + token_ids_1 + sep) * [0]