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# Create a custom architecture

An [`AutoClass`](model_doc/auto) automatically infers the model architecture and downloads pretrained configuration and weights. Generally, we recommend using an `AutoClass` to produce checkpoint-agnostic code. But users who want more control over specific model parameters can create a custom 🤗 Transformers model from just a few base classes. This could be particularly useful for anyone who is interested in studying, training or experimenting with a 🤗 Transformers model. In this guide, dive deeper into creating a custom model without an `AutoClass`. Learn how to:

- Load and customize a model configuration.
- Create a model architecture.
- Create a slow and fast tokenizer for text.
- Create a feature extractor for audio or image tasks.
- Create a processor for multimodal tasks.

## Configuration

A [configuration](main_classes/configuration) refers to a model's specific attributes. Each model configuration has different attributes; for instance, all NLP models have the `hidden_size`, `num_attention_heads`, `num_hidden_layers` and `vocab_size` attributes in common. These attributes specify the number of attention heads or hidden layers to construct a model with.

Get a closer look at [DistilBERT](model_doc/distilbert) by accessing [`DistilBertConfig`] to inspect it's attributes:

```py
>>> from transformers import DistilBertConfig

>>> config = DistilBertConfig()
>>> print(config)
DistilBertConfig {
  "activation": "gelu",
  "attention_dropout": 0.1,
  "dim": 768,
  "dropout": 0.1,
  "hidden_dim": 3072,
  "initializer_range": 0.02,
  "max_position_embeddings": 512,
  "model_type": "distilbert",
  "n_heads": 12,
  "n_layers": 6,
  "pad_token_id": 0,
  "qa_dropout": 0.1,
  "seq_classif_dropout": 0.2,
  "sinusoidal_pos_embds": false,
  "transformers_version": "4.16.2",
  "vocab_size": 30522
}
```

[`DistilBertConfig`] displays all the default attributes used to build a base [`DistilBertModel`]. All attributes are customizable, creating space for experimentation. For example, you can customize a default model to:

- Try a different activation function with the `activation` parameter.
- Use a higher dropout ratio for the attention probabilities with the `attention_dropout` parameter.

```py
>>> my_config = DistilBertConfig(activation="relu", attention_dropout=0.4)
>>> print(my_config)
DistilBertConfig {
  "activation": "relu",
  "attention_dropout": 0.4,
  "dim": 768,
  "dropout": 0.1,
  "hidden_dim": 3072,
  "initializer_range": 0.02,
  "max_position_embeddings": 512,
  "model_type": "distilbert",
  "n_heads": 12,
  "n_layers": 6,
  "pad_token_id": 0,
  "qa_dropout": 0.1,
  "seq_classif_dropout": 0.2,
  "sinusoidal_pos_embds": false,
  "transformers_version": "4.16.2",
  "vocab_size": 30522
}
```

Pretrained model attributes can be modified in the [`~PretrainedConfig.from_pretrained`] function:

```py
>>> my_config = DistilBertConfig.from_pretrained("distilbert-base-uncased", activation="relu", attention_dropout=0.4)
```

Once you are satisfied with your model configuration, you can save it with [`~PretrainedConfig.save_pretrained`]. Your configuration file is stored as a JSON file in the specified save directory:

```py
>>> my_config.save_pretrained(save_directory="./your_model_save_path")
```

To reuse the configuration file, load it with [`~PretrainedConfig.from_pretrained`]:

```py
>>> my_config = DistilBertConfig.from_pretrained("./your_model_save_path/my_config.json")
```

<Tip>

You can also save your configuration file as a dictionary or even just the difference between your custom configuration attributes and the default configuration attributes! See the [configuration](main_classes/configuration) documentation for more details.

</Tip>

## Model

The next step is to create a [model](main_classes/models). The model - also loosely referred to as the architecture - defines what each layer is doing and what operations are happening. Attributes like `num_hidden_layers` from the configuration are used to define the architecture. Every model shares the base class [`PreTrainedModel`] and a few common methods like resizing input embeddings and pruning self-attention heads. In addition, all models are also either a [`torch.nn.Module`](https://pytorch.org/docs/stable/generated/torch.nn.Module.html), [`tf.keras.Model`](https://www.tensorflow.org/api_docs/python/tf/keras/Model) or [`flax.linen.Module`](https://flax.readthedocs.io/en/latest/flax.linen.html#module) subclass. This means models are compatible with each of their respective framework's usage.

<frameworkcontent>
<pt>
Load your custom configuration attributes into the model:

```py
>>> from transformers import DistilBertModel

>>> my_config = DistilBertConfig.from_pretrained("./your_model_save_path/my_config.json")
>>> model = DistilBertModel(my_config)
```

This creates a model with random values instead of pretrained weights. You won't be able to use this model for anything useful yet until you train it. Training is a costly and time-consuming process. It is generally better to use a pretrained model to obtain better results faster, while using only a fraction of the resources required for training.

Create a pretrained model with [`~PreTrainedModel.from_pretrained`]:

```py
>>> model = DistilBertModel.from_pretrained("distilbert-base-uncased")
```

When you load pretrained weights, the default model configuration is automatically loaded if the model is provided by 🤗 Transformers. However, you can still replace - some or all of - the default model configuration attributes with your own if you'd like:

```py
>>> model = DistilBertModel.from_pretrained("distilbert-base-uncased", config=my_config)
```
</pt>
<tf>
Load your custom configuration attributes into the model:

```py
>>> from transformers import TFDistilBertModel

>>> my_config = DistilBertConfig.from_pretrained("./your_model_save_path/my_config.json")
>>> tf_model = TFDistilBertModel(my_config)
```

This creates a model with random values instead of pretrained weights. You won't be able to use this model for anything useful yet until you train it. Training is a costly and time-consuming process. It is generally better to use a pretrained model to obtain better results faster, while using only a fraction of the resources required for training.

Create a pretrained model with [`~TFPreTrainedModel.from_pretrained`]:

```py
>>> tf_model = TFDistilBertModel.from_pretrained("distilbert-base-uncased")
```

When you load pretrained weights, the default model configuration is automatically loaded if the model is provided by 🤗 Transformers. However, you can still replace - some or all of - the default model configuration attributes with your own if you'd like:

```py
>>> tf_model = TFDistilBertModel.from_pretrained("distilbert-base-uncased", config=my_config)
```
</tf>
</frameworkcontent>

### Model heads

At this point, you have a base DistilBERT model which outputs the *hidden states*. The hidden states are passed as inputs to a model head to produce the final output. 🤗 Transformers provides a different model head for each task as long as a model supports the task (i.e., you can't use DistilBERT for a sequence-to-sequence task like translation).

<frameworkcontent>
<pt>
For example, [`DistilBertForSequenceClassification`] is a base DistilBERT model with a sequence classification head. The sequence classification head is a linear layer on top of the pooled outputs.

```py
>>> from transformers import DistilBertForSequenceClassification

>>> model = DistilBertForSequenceClassification.from_pretrained("distilbert-base-uncased")
```

Easily reuse this checkpoint for another task by switching to a different model head. For a question answering task, you would use the [`DistilBertForQuestionAnswering`] model head. The question answering head is similar to the sequence classification head except it is a linear layer on top of the hidden states output.

```py
>>> from transformers import DistilBertForQuestionAnswering

>>> model = DistilBertForQuestionAnswering.from_pretrained("distilbert-base-uncased")
```
</pt>
<tf>
For example, [`TFDistilBertForSequenceClassification`] is a base DistilBERT model with a sequence classification head. The sequence classification head is a linear layer on top of the pooled outputs.

```py
>>> from transformers import TFDistilBertForSequenceClassification

>>> tf_model = TFDistilBertForSequenceClassification.from_pretrained("distilbert-base-uncased")
```

Easily reuse this checkpoint for another task by switching to a different model head. For a question answering task, you would use the [`TFDistilBertForQuestionAnswering`] model head. The question answering head is similar to the sequence classification head except it is a linear layer on top of the hidden states output.

```py
>>> from transformers import TFDistilBertForQuestionAnswering

>>> tf_model = TFDistilBertForQuestionAnswering.from_pretrained("distilbert-base-uncased")
```
</tf>
</frameworkcontent>

## Tokenizer

The last base class you need before using a model for textual data is a [tokenizer](main_classes/tokenizer) to convert raw text to tensors. There are two types of tokenizers you can use with 🤗 Transformers:

- [`PreTrainedTokenizer`]: a Python implementation of a tokenizer.
- [`PreTrainedTokenizerFast`]: a tokenizer from our Rust-based [🤗 Tokenizer](https://huggingface.co/docs/tokenizers/python/latest/) library. This tokenizer type is significantly faster - especially during batch tokenization - due to it's Rust implementation. The fast tokenizer also offers additional methods like *offset mapping* which maps tokens to their original words or characters.

Both tokenizers support common methods such as encoding and decoding, adding new tokens, and managing special tokens.

<Tip warning={true}>

Not every model supports a fast tokenizer. Take a look at this [table](index#supported-frameworks) to check if a model has fast tokenizer support.

</Tip>

If you trained your own tokenizer, you can create one from your *vocabulary* file:

```py
>>> from transformers import DistilBertTokenizer

>>> my_tokenizer = DistilBertTokenizer(vocab_file="my_vocab_file.txt", do_lower_case=False, padding_side="left")
```

It is important to remember the vocabulary from a custom tokenizer will be different from the vocabulary generated by a pretrained model's tokenizer. You need to use a pretrained model's vocabulary if you are using a pretrained model, otherwise the inputs won't make sense. Create a tokenizer with a pretrained model's vocabulary with the [`DistilBertTokenizer`] class:

```py
>>> from transformers import DistilBertTokenizer

>>> slow_tokenizer = DistilBertTokenizer.from_pretrained("distilbert-base-uncased")
```

Create a fast tokenizer with the [`DistilBertTokenizerFast`] class:

```py
>>> from transformers import DistilBertTokenizerFast

>>> fast_tokenizer = DistilBertTokenizerFast.from_pretrained("distilbert-base-uncased")
```

<Tip>

By default, [`AutoTokenizer`] will try to load a fast tokenizer. You can disable this behavior by setting `use_fast=False` in `from_pretrained`.

</Tip>

## Feature Extractor

A feature extractor processes audio or image inputs. It inherits from the base [`~feature_extraction_utils.FeatureExtractionMixin`] class, and may also inherit from the [`ImageFeatureExtractionMixin`] class for processing image features or the [`SequenceFeatureExtractor`] class for processing audio inputs.

Depending on whether you are working on an audio or vision task, create a feature extractor associated with the model you're using. For example, create a default [`ViTFeatureExtractor`] if you are using [ViT](model_doc/vit) for image classification:

```py
>>> from transformers import ViTFeatureExtractor

>>> vit_extractor = ViTFeatureExtractor()
>>> print(vit_extractor)
ViTFeatureExtractor {
  "do_normalize": true,
  "do_resize": true,
  "feature_extractor_type": "ViTFeatureExtractor",
  "image_mean": [
    0.5,
    0.5,
    0.5
  ],
  "image_std": [
    0.5,
    0.5,
    0.5
  ],
  "resample": 2,
  "size": 224
}
```

<Tip>

If you aren't looking for any customization, just use the `from_pretrained` method to load a model's default feature extractor parameters.

</Tip>

Modify any of the [`ViTFeatureExtractor`] parameters to create your custom feature extractor:

```py
>>> from transformers import ViTFeatureExtractor

>>> my_vit_extractor = ViTFeatureExtractor(resample="PIL.Image.BOX", do_normalize=False, image_mean=[0.3, 0.3, 0.3])
>>> print(my_vit_extractor)
ViTFeatureExtractor {
  "do_normalize": false,
  "do_resize": true,
  "feature_extractor_type": "ViTFeatureExtractor",
  "image_mean": [
    0.3,
    0.3,
    0.3
  ],
  "image_std": [
    0.5,
    0.5,
    0.5
  ],
  "resample": "PIL.Image.BOX",
  "size": 224
}
```

For audio inputs, you can create a [`Wav2Vec2FeatureExtractor`] and customize the parameters in a similar way:

```py
>>> from transformers import Wav2Vec2FeatureExtractor

>>> w2v2_extractor = Wav2Vec2FeatureExtractor()
>>> print(w2v2_extractor)
Wav2Vec2FeatureExtractor {
  "do_normalize": true,
  "feature_extractor_type": "Wav2Vec2FeatureExtractor",
  "feature_size": 1,
  "padding_side": "right",
  "padding_value": 0.0,
  "return_attention_mask": false,
  "sampling_rate": 16000
}
```

## Processor

For models that support multimodal tasks, 🤗 Transformers offers a processor class that conveniently wraps a feature extractor and tokenizer into a single object. For example, let's use the [`Wav2Vec2Processor`] for an automatic speech recognition task (ASR). ASR transcribes audio to text, so you will need a feature extractor and a tokenizer.

Create a feature extractor to handle the audio inputs:

```py
>>> from transformers import Wav2Vec2FeatureExtractor

>>> feature_extractor = Wav2Vec2FeatureExtractor(padding_value=1.0, do_normalize=True)
```

Create a tokenizer to handle the text inputs:

```py
>>> from transformers import Wav2Vec2CTCTokenizer

>>> tokenizer = Wav2Vec2CTCTokenizer(vocab_file="my_vocab_file.txt")
```

Combine the feature extractor and tokenizer in [`Wav2Vec2Processor`]:

```py
>>> from transformers import Wav2Vec2Processor

>>> processor = Wav2Vec2Processor(feature_extractor=feature_extractor, tokenizer=tokenizer)
```

With two basic classes - configuration and model - and an additional preprocessing class (tokenizer, feature extractor, or processor), you can create any of the models supported by 🤗 Transformers. Each of these base classes are configurable, allowing you to use the specific attributes you want. You can easily setup a model for training or modify an existing pretrained model to fine-tune.