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272 lines
9.9 KiB
Plaintext
<!--Copyright 2022 The HuggingFace Team. All rights reserved.
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Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
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the License. You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
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an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
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specific language governing permissions and limitations under the License.
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-->
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# Token classification
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<Youtube id="wVHdVlPScxA"/>
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Token classification assigns a label to individual tokens in a sentence. One of the most common token classification tasks is Named Entity Recognition (NER). NER attempts to find a label for each entity in a sentence, such as a person, location, or organization.
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This guide will show you how to fine-tune [DistilBERT](https://huggingface.co/distilbert-base-uncased) on the [WNUT 17](https://huggingface.co/datasets/wnut_17) dataset to detect new entities.
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<Tip>
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See the token classification [task page](https://huggingface.co/tasks/token-classification) for more information about other forms of token classification and their associated models, datasets, and metrics.
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</Tip>
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## Load WNUT 17 dataset
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Load the WNUT 17 dataset from the 🤗 Datasets library:
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```py
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>>> from datasets import load_dataset
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>>> wnut = load_dataset("wnut_17")
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```
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Then take a look at an example:
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```py
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>>> wnut["train"][0]
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{'id': '0',
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'ner_tags': [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 7, 8, 8, 0, 7, 0, 0, 0, 0, 0, 0, 0, 0],
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'tokens': ['@paulwalk', 'It', "'s", 'the', 'view', 'from', 'where', 'I', "'m", 'living', 'for', 'two', 'weeks', '.', 'Empire', 'State', 'Building', '=', 'ESB', '.', 'Pretty', 'bad', 'storm', 'here', 'last', 'evening', '.']
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}
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```
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Each number in `ner_tags` represents an entity. Convert the number to a label name for more information:
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```py
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>>> label_list = wnut["train"].features[f"ner_tags"].feature.names
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>>> label_list
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[
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"O",
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"B-corporation",
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"I-corporation",
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"B-creative-work",
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"I-creative-work",
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"B-group",
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"I-group",
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"B-location",
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"I-location",
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"B-person",
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"I-person",
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"B-product",
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"I-product",
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]
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```
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The `ner_tag` describes an entity, such as a corporation, location, or person. The letter that prefixes each `ner_tag` indicates the token position of the entity:
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- `B-` indicates the beginning of an entity.
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- `I-` indicates a token is contained inside the same entity (e.g., the `State` token is a part of an entity like
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`Empire State Building`).
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- `0` indicates the token doesn't correspond to any entity.
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## Preprocess
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<Youtube id="iY2AZYdZAr0"/>
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Load the DistilBERT tokenizer to process the `tokens`:
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```py
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>>> from transformers import AutoTokenizer
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>>> tokenizer = AutoTokenizer.from_pretrained("distilbert-base-uncased")
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```
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Since the input has already been split into words, set `is_split_into_words=True` to tokenize the words into subwords:
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```py
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>>> tokenized_input = tokenizer(example["tokens"], is_split_into_words=True)
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>>> tokens = tokenizer.convert_ids_to_tokens(tokenized_input["input_ids"])
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>>> tokens
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['[CLS]', '@', 'paul', '##walk', 'it', "'", 's', 'the', 'view', 'from', 'where', 'i', "'", 'm', 'living', 'for', 'two', 'weeks', '.', 'empire', 'state', 'building', '=', 'es', '##b', '.', 'pretty', 'bad', 'storm', 'here', 'last', 'evening', '.', '[SEP]']
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```
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Adding the special tokens `[CLS]` and `[SEP]` and subword tokenization creates a mismatch between the input and labels. A single word corresponding to a single label may be split into two subwords. You will need to realign the tokens and labels by:
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1. Mapping all tokens to their corresponding word with the [`word_ids`](https://huggingface.co/docs/tokenizers/python/latest/api/reference.html#tokenizers.Encoding.word_ids) method.
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2. Assigning the label `-100` to the special tokens `[CLS]` and `[SEP]` so the PyTorch loss function ignores
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them.
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3. Only labeling the first token of a given word. Assign `-100` to other subtokens from the same word.
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Here is how you can create a function to realign the tokens and labels, and truncate sequences to be no longer than DistilBERT's maximum input length::
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```py
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>>> def tokenize_and_align_labels(examples):
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... tokenized_inputs = tokenizer(examples["tokens"], truncation=True, is_split_into_words=True)
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... labels = []
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... for i, label in enumerate(examples[f"ner_tags"]):
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... word_ids = tokenized_inputs.word_ids(batch_index=i) # Map tokens to their respective word.
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... previous_word_idx = None
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... label_ids = []
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... for word_idx in word_ids: # Set the special tokens to -100.
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... if word_idx is None:
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... label_ids.append(-100)
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... elif word_idx != previous_word_idx: # Only label the first token of a given word.
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... label_ids.append(label[word_idx])
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... else:
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... label_ids.append(-100)
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... previous_word_idx = word_idx
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... labels.append(label_ids)
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... tokenized_inputs["labels"] = labels
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... return tokenized_inputs
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```
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Use 🤗 Datasets [`map`](https://huggingface.co/docs/datasets/package_reference/main_classes.html#datasets.Dataset.map) function to tokenize and align the labels over the entire dataset. You can speed up the `map` function by setting `batched=True` to process multiple elements of the dataset at once:
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```py
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>>> tokenized_wnut = wnut.map(tokenize_and_align_labels, batched=True)
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```
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Use [`DataCollatorForTokenClassification`] to create a batch of examples. It will also *dynamically pad* your text and labels to the length of the longest element in its batch, so they are a uniform length. While it is possible to pad your text in the `tokenizer` function by setting `padding=True`, dynamic padding is more efficient.
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<frameworkcontent>
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<pt>
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```py
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>>> from transformers import DataCollatorForTokenClassification
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>>> data_collator = DataCollatorForTokenClassification(tokenizer=tokenizer)
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```
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</pt>
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<tf>
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```py
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>>> from transformers import DataCollatorForTokenClassification
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>>> data_collator = DataCollatorForTokenClassification(tokenizer=tokenizer, return_tensors="tf")
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```
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</tf>
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</frameworkcontent>
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## Train
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<frameworkcontent>
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<pt>
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Load DistilBERT with [`AutoModelForTokenClassification`] along with the number of expected labels:
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```py
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>>> from transformers import AutoModelForTokenClassification, TrainingArguments, Trainer
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>>> model = AutoModelForTokenClassification.from_pretrained("distilbert-base-uncased", num_labels=14)
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```
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<Tip>
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If you aren't familiar with fine-tuning a model with the [`Trainer`], take a look at the basic tutorial [here](../training#finetune-with-trainer)!
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</Tip>
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At this point, only three steps remain:
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1. Define your training hyperparameters in [`TrainingArguments`].
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2. Pass the training arguments to [`Trainer`] along with the model, dataset, tokenizer, and data collator.
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3. Call [`~Trainer.train`] to fine-tune your model.
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```py
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>>> training_args = TrainingArguments(
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... output_dir="./results",
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... evaluation_strategy="epoch",
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... learning_rate=2e-5,
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... per_device_train_batch_size=16,
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... per_device_eval_batch_size=16,
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... num_train_epochs=3,
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... weight_decay=0.01,
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... )
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>>> trainer = Trainer(
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... model=model,
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... args=training_args,
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... train_dataset=tokenized_wnut["train"],
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... eval_dataset=tokenized_wnut["test"],
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... tokenizer=tokenizer,
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... data_collator=data_collator,
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... )
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>>> trainer.train()
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```
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</pt>
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<tf>
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To fine-tune a model in TensorFlow, start by converting your datasets to the `tf.data.Dataset` format with [`to_tf_dataset`](https://huggingface.co/docs/datasets/package_reference/main_classes.html#datasets.Dataset.to_tf_dataset). Specify inputs and labels in `columns`, whether to shuffle the dataset order, batch size, and the data collator:
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```py
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>>> tf_train_set = tokenized_wnut["train"].to_tf_dataset(
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... columns=["attention_mask", "input_ids", "labels"],
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... shuffle=True,
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... batch_size=16,
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... collate_fn=data_collator,
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... )
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>>> tf_validation_set = tokenized_wnut["validation"].to_tf_dataset(
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... columns=["attention_mask", "input_ids", "labels"],
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... shuffle=False,
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... batch_size=16,
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... collate_fn=data_collator,
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... )
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```
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<Tip>
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If you aren't familiar with fine-tuning a model with Keras, take a look at the basic tutorial [here](training#finetune-with-keras)!
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</Tip>
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Set up an optimizer function, learning rate schedule, and some training hyperparameters:
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```py
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>>> from transformers import create_optimizer
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>>> batch_size = 16
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>>> num_train_epochs = 3
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>>> num_train_steps = (len(tokenized_wnut["train"]) // batch_size) * num_train_epochs
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>>> optimizer, lr_schedule = create_optimizer(
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... init_lr=2e-5,
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... num_train_steps=num_train_steps,
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... weight_decay_rate=0.01,
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... num_warmup_steps=0,
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... )
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```
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Load DistilBERT with [`TFAutoModelForTokenClassification`] along with the number of expected labels:
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```py
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>>> from transformers import TFAutoModelForTokenClassification
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>>> model = TFAutoModelForTokenClassification.from_pretrained("distilbert-base-uncased", num_labels=2)
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```
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Configure the model for training with [`compile`](https://keras.io/api/models/model_training_apis/#compile-method):
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```py
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>>> import tensorflow as tf
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>>> model.compile(optimizer=optimizer)
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```
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Call [`fit`](https://keras.io/api/models/model_training_apis/#fit-method) to fine-tune the model:
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```py
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>>> model.fit(x=tf_train_set, validation_data=tf_validation_set, epochs=3)
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```
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</tf>
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</frameworkcontent>
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<Tip>
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For a more in-depth example of how to fine-tune a model for token classification, take a look at the corresponding
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[PyTorch notebook](https://colab.research.google.com/github/huggingface/notebooks/blob/master/examples/token_classification.ipynb)
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or [TensorFlow notebook](https://colab.research.google.com/github/huggingface/notebooks/blob/master/examples/token_classification-tf.ipynb).
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</Tip> |