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# Loading models

Transformers provides many pretrained models that are ready to use with a single line of code. It requires a model class and the [`~PreTrainedModel.from_pretrained`] method.

Call [`~PreTrainedModel.from_pretrained`] to download and load a model's weights and configuration stored on the Hugging Face [Hub](https://hf.co/models).

> [!TIP]
> The [`~PreTrainedModel.from_pretrained`] method loads weights stored in the [safetensors](https://hf.co/docs/safetensors/index) file format if they're available. Traditionally, PyTorch model weights are serialized with the [pickle](https://docs.python.org/3/library/pickle.html) utility which is known to be unsecure. Safetensor files are more secure and faster to load.

```py
from transformers import AutoModelForCausalLM

model = AutoModelForCausalLM.from_pretrained("meta-llama/Llama-2-7b-hf", device_map="auto")
```

This guide explains how models are loaded, the different ways you can load a model, how to overcome memory issues for really big models, and how to load custom models.

## Models and configurations

All models have a `configuration.py` file with specific attributes like the number of hidden layers, vocabulary size, activation function, and more. You'll also find a `modeling.py` file that defines the layers and mathematical operations taking place inside each layer. The `modeling.py` file takes the model attributes in `configuration.py` and builds the model accordingly. At this point, you have a model with random weights that needs to be trained to output meaningful results.

<!-- insert diagram of model and configuration -->

> [!TIP]
> An *architecture* refers to the model's skeleton and a *checkpoint* refers to the model's weights for a given architecture. For example, [BERT](./model_doc/bert) is an architecture while [google-bert/bert-base-uncased](https://huggingface.co/google-bert/bert-base-uncased) is a checkpoint. You'll see the term *model* used interchangeably with architecture and checkpoint.

There are two general types of models you can load:

1. A barebones model, like [`AutoModel`] or [`LlamaModel`], that outputs hidden states.
2. A model with a specific *head* attached, like [`AutoModelForCausalLM`] or [`LlamaForCausalLM`], for performing specific tasks.

## Model classes

To get a pretrained model, you need to load the weights into the model. This is done by calling [`~PreTrainedModel.from_pretrained`] which accepts weights from the Hugging Face Hub or a local directory.

There are two model classes, the [AutoModel](./model_doc/auto) class and a model-specific class.

<hfoptions id="model-classes">
<hfoption id="AutoModel">

<Youtube id="AhChOFRegn4"/>

The [AutoModel](./model_doc/auto) class is a convenient way to load an architecture without needing to know the exact model class name because there are many models available. It automatically selects the correct model class based on the configuration file. You only need to know the task and checkpoint you want to use.

Easily switch between models or tasks, as long as the architecture is supported for a given task.

For example, the same model can be used for separate tasks.

```py
from transformers import AutoModelForCausalLM, AutoModelForSequenceClassification, AutoModelForQuestionAnswering

# use the same API for 3 different tasks
model = AutoModelForCausalLM.from_pretrained("meta-llama/Llama-2-7b-hf")
model = AutoModelForSequenceClassification.from_pretrained("meta-llama/Llama-2-7b-hf")
model = AutoModelForQuestionAnswering.from_pretrained("meta-llama/Llama-2-7b-hf")
```

In other cases, you may want to quickly try out several different models for a task.

```py
from transformers import AutoModelForCausalLM

# use the same API to load 3 different models
model = AutoModelForCausalLM.from_pretrained("meta-llama/Llama-2-7b-hf")
model = AutoModelForCausalLM.from_pretrained("mistralai/Mistral-7B-v0.1")
model = AutoModelForCausalLM.from_pretrained("google/gemma-7b")
```

</hfoption>
<hfoption id="model-specific class">

The [AutoModel](./model_doc/auto) class builds on top of model-specific classes. All model classes that support a specific task are mapped to their respective `AutoModelFor` task class.

If you already know which model class you want to use, then you could use its model-specific class directly.

```py
from transformers import LlamaModel, LlamaForCausalLM

model = LlamaForCausalLM.from_pretrained("meta-llama/Llama-2-7b-hf")
```

</hfoption>
</hfoptions>

## Large models

Large pretrained models require a lot of memory to load. The loading process involves:

1. creating a model with random weights
2. loading the pretrained weights
3. placing the pretrained weights on the model

You need enough memory to hold two copies of the model weights (random and pretrained) which may not be possible depending on your hardware. In distributed training environments, this is even more challenging because each process loads a pretrained model.

Transformers reduces some of these memory-related challenges with fast initialization, sharded checkpoints, Accelerate's [Big Model Inference](https://hf.co/docs/accelerate/usage_guides/big_modeling) feature, and supporting lower bit data types.

### Sharded checkpoints

[`~PreTrainedModel.save_pretrained`] automatically shards checkpoints larger than 50GB. This keeps shard counts low for large models and simplifies file management.

Parameters load in parallel and peak memory only depends on model size. Use `max_shard_size` in [`~PreTrainedModel.save_pretrained`] to set the maximum checkpoint size before sharding.

> [!NOTE]
> Memory usage for models requiring dynamic weight conversion depends on the model size and the size of the largest parameters in a single conversion. This typically applies to mixture-of-experts (MoE) models where the memory usage is the model size plus the number of experts on one layer. Refer to the [dynamic weight loader](./weightconverter#fast-and-efficient-model-loading) guide to learn more about how models are loaded.

[`~PreTrainedModel.save_pretrained`] also creates an index file mapping parameter names to their shard files. The index contains two keys, `metadata` and `weight_map`.

```py
import json

with tempfile.TemporaryDirectory() as tmp_dir:
    model.save_pretrained(tmp_dir, max_shard_size="50GB")
    with open(os.path.join(tmp_dir, "model.safetensors.index.json"), "r") as f:
        index = json.load(f)

print(index.keys())
```

`metadata` stores the total model size.

```py
index["metadata"]
{'total_size': 28966928384}
```

`weight_map` maps each parameter to its shard file.

```py
index["weight_map"]
{'lm_head.weight': 'model-00006-of-00006.safetensors',
 'model.embed_tokens.weight': 'model-00001-of-00006.safetensors',
 'model.layers.0.input_layernorm.weight': 'model-00001-of-00006.safetensors',
 'model.layers.0.mlp.down_proj.weight': 'model-00001-of-00006.safetensors',
 ...
}
```

### Big Model Inference

> [!TIP]
> Make sure you have Accelerate v0.9.0 and PyTorch v1.9.0 or later installed to use this feature!

<Youtube id="MWCSGj9jEAo"/>

[`~PreTrainedModel.from_pretrained`] is supercharged with Accelerate's [Big Model Inference](https://hf.co/docs/accelerate/usage_guides/big_modeling) feature.

Big Model Inference creates a *model skeleton* on the PyTorch [meta](https://pytorch.org/docs/main/meta.html) device. The meta device doesn't store any real data, only the metadata.

Randomly initialized weights are only created when the pretrained weights are loaded to avoid maintaining two copies of the model in memory at the same time. The maximum memory usage is only the size of the model.

> [!TIP]
> Learn more about device placement in [Designing a device map](https://hf.co/docs/accelerate/v0.33.0/en/concept_guides/big_model_inference#designing-a-device-map).

Big Model Inference's second feature relates to how weights are loaded and dispatched in the model skeleton. Model weights are dispatched across all available devices, starting with the fastest device (usually the GPU) and then offloading any remaining weights to slower devices (CPU and hard drive).

Both features combined reduces memory usage and loading times for big pretrained models.

Set [device_map](https://github.com/huggingface/transformers/blob/026a173a64372e9602a16523b8fae9de4b0ff428/src/transformers/modeling_utils.py#L3061) to `"auto"` to enable Big Model Inference.

```py
from transformers import AutoModelForCausalLM

model = AutoModelForCausalLM.from_pretrained("google/gemma-7b", device_map="auto")
```

You can also manually assign layers to a device in `device_map`. It should map all model parameters to a device, but you don't have to detail where all the submodules of a layer go if the entire layer is on the same device.

Access the `hf_device_map` attribute to see how a model is distributed across devices.

```py
device_map = {"model.layers.1": 0, "model.layers.14": 1, "model.layers.31": "cpu", "lm_head": "disk"}
model.hf_device_map
```

### Model data type

The `dtype` argument controls the PyTorch [dtype](https://pytorch.org/docs/stable/tensor_attributes.html#torch.dtype) used to instantiate model weights. By default, Transformers loads weights with the `dtype` or legacy `torch_dtype` value from `config.json`. If `config.json` doesn't include either value, Transformers uses the dtype of the first floating-point weight in the checkpoint.

Override the default by passing a specific dtype.

```py
import torch
from transformers import AutoModelForCausalLM

# specific dtype
model = AutoModelForCausalLM.from_pretrained("google/gemma-3-1b-it", dtype=torch.float16)
```

[`AutoConfig`] also accepts `dtype` for models instantiated from scratch.

```py
import torch
from transformers import AutoConfig, AutoModel

my_config = AutoConfig.from_pretrained("google/gemma-2b", dtype=torch.float16)
model = AutoModel.from_config(my_config)
```

## Custom models

Custom models builds on Transformers' configuration and modeling classes, supports the [AutoClass](#autoclass) API, and are loaded with [`~PreTrainedModel.from_pretrained`]. The difference is that the modeling code is *not* from Transformers.

Take extra precaution when loading a custom model. While the Hub includes [malware scanning](https://hf.co/docs/hub/security-malware#malware-scanning) for every repository, you should still be careful to avoid inadvertently executing malicious code.

Set `trust_remote_code=True` in [`~PreTrainedModel.from_pretrained`] to load a custom model.

```py
from transformers import AutoModelForImageClassification

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

As an extra layer of security, load a custom model from a specific revision to avoid loading model code that may have changed. The commit hash can be copied from the models [commit history](https://hf.co/sgugger/custom-resnet50d/commits/main).

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

Refer to the [Customize models](./custom_models) guide for more information.
