Building custom models

The 🤗 Transformers library is designed to be easily extensible. Every model is fully coded in a given subfolder of the repository with no abstraction, so you can easily copy a modeling file and tweak it to your needs.

If you are writing a brand new model, it might be easier to start from scratch. In this tutorial, we will show you how to write a custom model and its configuration so it can be used inside Transformers, and how you can share it with the community (with the code it relies on) so that anyone can use it, even if it’s not present in the 🤗 Transformers library. We’ll see how to build upon transformers and extend the framework with your hooks and custom code.

We will illustrate all of this on a ResNet model, by wrapping the ResNet class of the timm library into a PreTrainedModel.

Writing a custom configuration

Before we dive into the model, let’s first write its configuration. The configuration of a model is an object that will contain all the necessary information to build the model. As we will see in the next section, the model can only take a config to be initialized, so we really need that object to be as complete as possible.

In our example, we will take a couple of arguments of the ResNet class that we might want to tweak. Different configurations will then give us the different types of ResNets that are possible. We then just store those arguments, after checking the validity of a few of them.

from transformers import PretrainedConfig
from typing import List


class ResnetConfig(PretrainedConfig):
    model_type = "resnet"

    def __init__(
        self,
        block_type="bottleneck",
        layers: List[int] = [3, 4, 6, 3],
        num_classes: int = 1000,
        input_channels: int = 3,
        cardinality: int = 1,
        base_width: int = 64,
        stem_width: int = 64,
        stem_type: str = "",
        avg_down: bool = False,
        **kwargs,
    ):
        if block_type not in ["basic", "bottleneck"]:
            raise ValueError(f"`block_type` must be 'basic' or bottleneck', got {block_type}.")
        if stem_type not in ["", "deep", "deep-tiered"]:
            raise ValueError(f"`stem_type` must be '', 'deep' or 'deep-tiered', got {stem_type}.")

        self.block_type = block_type
        self.layers = layers
        self.num_classes = num_classes
        self.input_channels = input_channels
        self.cardinality = cardinality
        self.base_width = base_width
        self.stem_width = stem_width
        self.stem_type = stem_type
        self.avg_down = avg_down
        super().__init__(**kwargs)

The three important things to remember when writing you own configuration are the following:

• you have to inherit from PretrainedConfig,
• the __init__ of your PretrainedConfig must accept any kwargs,
• those kwargs need to be passed to the superclass __init__.

The inheritance is to make sure you get all the functionality from the 🤗 Transformers library, while the two other constraints come from the fact a PretrainedConfig has more fields than the ones you are setting. When reloading a config with the from_pretrained method, those fields need to be accepted by your config and then sent to the superclass.

Defining a model_type for your configuration (here model_type="resnet") is not mandatory, unless you want to register your model with the auto classes (see last section).

With this done, you can easily create and save your configuration like you would do with any other model config of the library. Here is how we can create a resnet50d config and save it:

resnet50d_config = ResnetConfig(block_type="bottleneck", stem_width=32, stem_type="deep", avg_down=True)
resnet50d_config.save_pretrained("custom-resnet")

This will save a file named config.json inside the folder custom-resnet. You can then reload your config with the from_pretrained method:

resnet50d_config = ResnetConfig.from_pretrained("custom-resnet")

You can also use any other method of the PretrainedConfig class, like push_to_hub() to directly upload your config to the Hub.

Writing a custom model

Now that we have our ResNet configuration, we can go on writing the model. We will actually write two: one that extracts the hidden features from a batch of images (like BertModel) and one that is suitable for image classification (like BertForSequenceClassification).

As we mentioned before, we’ll only write a loose wrapper of the model to keep it simple for this example. The only thing we need to do before writing this class is a map between the block types and actual block classes. Then the model is defined from the configuration by passing everything to the ResNet class:

from transformers import PreTrainedModel
from timm.models.resnet import BasicBlock, Bottleneck, ResNet
from .configuration_resnet import ResnetConfig


BLOCK_MAPPING = {"basic": BasicBlock, "bottleneck": Bottleneck}


class ResnetModel(PreTrainedModel):
    config_class = ResnetConfig

    def __init__(self, config):
        super().__init__(config)
        block_layer = BLOCK_MAPPING[config.block_type]
        self.model = ResNet(
            block_layer,
            config.layers,
            num_classes=config.num_classes,
            in_chans=config.input_channels,
            cardinality=config.cardinality,
            base_width=config.base_width,
            stem_width=config.stem_width,
            stem_type=config.stem_type,
            avg_down=config.avg_down,
        )

    def forward(self, tensor):
        return self.model.forward_features(tensor)

For the model that will classify images, we just change the forward method:

import torch


class ResnetModelForImageClassification(PreTrainedModel):
    config_class = ResnetConfig

    def __init__(self, config):
        super().__init__(config)
        block_layer = BLOCK_MAPPING[config.block_type]
        self.model = ResNet(
            block_layer,
            config.layers,
            num_classes=config.num_classes,
            in_chans=config.input_channels,
            cardinality=config.cardinality,
            base_width=config.base_width,
            stem_width=config.stem_width,
            stem_type=config.stem_type,
            avg_down=config.avg_down,
        )

    def forward(self, tensor, labels=None):
        logits = self.model(tensor)
        if labels is not None:
            loss = torch.nn.cross_entropy(logits, labels)
            return {"loss": loss, "logits": logits}
        return {"logits": logits}

In both cases, notice how we inherit from PreTrainedModel and call the superclass initialization with the config (a bit like when you write a regular torch.nn.Module). The line that sets the config_class is not mandatory, unless you want to register your model with the auto classes (see last section).

You can have your model return anything you want, but returning a dictionary like we did for ResnetModelForImageClassification, with the loss included when labels are passed, will make your model directly usable inside the Trainer class. Using another output format is fine as long as you are planning on using your own training loop or another library for training.

Now that we have our model class, let’s create one:

resnet50d = ResnetModelForImageClassification(resnet50d_config)

Again, you can use any of the methods of PreTrainedModel, like save_pretrained() or push_to_hub(). We will use the second in the next section, and see how to push the model weights with the code of our model. But first, let’s load some pretrained weights inside our model.

In your own use case, you will probably be training your custom model on your own data. To go fast for this tutorial, we will use the pretrained version of the resnet50d. Since our model is just a wrapper around it, it’s going to be easy to transfer those weights:

import timm

pretrained_model = timm.create_model("resnet50d", pretrained=True)
resnet50d.model.load_state_dict(pretrained_model.state_dict())

Now let’s see how to make sure that when we do save_pretrained() or push_to_hub(), the code of the model is saved.

Registering a model with custom code to the auto classes

If you are writing a library that extends 🤗 Transformers, you may want to extend the auto classes to include your own model. This is different from pushing the code to the Hub in the sense that users will need to import your library to get the custom models (contrarily to automatically downloading the model code from the Hub).

As long as your config has a model_type attribute that is different from existing model types, and that your model classes have the right config_class attributes, you can just add them to the auto classes like this:

from transformers import AutoConfig, AutoModel, AutoModelForImageClassification

AutoConfig.register("resnet", ResnetConfig)
AutoModel.register(ResnetConfig, ResnetModel)
AutoModelForImageClassification.register(ResnetConfig, ResnetModelForImageClassification)

Note that the first argument used when registering your custom config to AutoConfig needs to match the model_type of your custom config, and the first argument used when registering your custom models to any auto model class needs to match the config_class of those models.

Sending the code to the Hub

First, make sure your model is fully defined in a .py file. It can rely on relative imports to some other files as long as all the files are in the same directory (we don’t support submodules for this feature yet). For our example, we’ll define a modeling_resnet.py file and a configuration_resnet.py file in a folder of the current working directory named resnet_model. The configuration file contains the code for ResnetConfig and the modeling file contains the code of ResnetModel and ResnetModelForImageClassification.

.
└── resnet_model
    ├── __init__.py
    ├── configuration_resnet.py
    └── modeling_resnet.py

The __init__.py can be empty, it’s just there so that Python detects resnet_model can be use as a module.

Note that you can re-use (or subclass) an existing configuration/model.

To share your model with the community, follow those steps: first import the ResNet model and config from the newly created files:

from resnet_model.configuration_resnet import ResnetConfig
from resnet_model.modeling_resnet import ResnetModel, ResnetModelForImageClassification

Then you have to tell the library you want to copy the code files of those objects when using the save_pretrained method and properly register them with a given Auto class (especially for models), just run:

ResnetConfig.register_for_auto_class()
ResnetModel.register_for_auto_class("AutoModel")
ResnetModelForImageClassification.register_for_auto_class("AutoModelForImageClassification")

Note that there is no need to specify an auto class for the configuration (there is only one auto class for them, AutoConfig) but it’s different for models. Your custom model could be suitable for many different tasks, so you have to specify which one of the auto classes is the correct one for your model.

"auto_map": {     
	"AutoConfig": "<your-repo-name>--<config-name>",     
	"AutoModel": "<your-repo-name>--<config-name>",
	"AutoModelFor<Task>": "<your-repo-name>--<config-name>",    
},

Next, let’s create the config and models as we did before:

resnet50d_config = ResnetConfig(block_type="bottleneck", stem_width=32, stem_type="deep", avg_down=True)
resnet50d = ResnetModelForImageClassification(resnet50d_config)

pretrained_model = timm.create_model("resnet50d", pretrained=True)
resnet50d.model.load_state_dict(pretrained_model.state_dict())

Now to send the model to the Hub, make sure you are logged in. Either run in your terminal:

huggingface-cli login

or from a notebook:

from huggingface_hub import notebook_login

notebook_login()

You can then push to your own namespace (or an organization you are a member of) like this:

resnet50d.push_to_hub("custom-resnet50d")

On top of the modeling weights and the configuration in json format, this also copied the modeling and configuration .py files in the folder custom-resnet50d and uploaded the result to the Hub. You can check the result in this model repo.

See the sharing tutorial for more information on the push to Hub method.

Using a model with custom code

You can use any configuration, model or tokenizer with custom code files in its repository with the auto-classes and the from_pretrained method. All files and code uploaded to the Hub are scanned for malware (refer to the Hub security documentation for more information), but you should still review the model code and author to avoid executing malicious code on your machine. Set trust_remote_code=True to use a model with custom code:

from transformers import AutoModelForImageClassification

model = AutoModelForImageClassification.from_pretrained("sgugger/custom-resnet50d", trust_remote_code=True)

It is also strongly encouraged to pass a commit hash as a revision to make sure the author of the models did not update the code with some malicious new lines (unless you fully trust the authors of the models).

commit_hash = "ed94a7c6247d8aedce4647f00f20de6875b5b292"
model = AutoModelForImageClassification.from_pretrained(
    "sgugger/custom-resnet50d", trust_remote_code=True, revision=commit_hash
)

Note that when browsing the commit history of the model repo on the Hub, there is a button to easily copy the commit hash of any commit.