transformers/docs/source/en/main_classes/model.mdx

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# Models

The base classes [`PreTrainedModel`], [`TFPreTrainedModel`], and
[`FlaxPreTrainedModel`] implement the common methods for loading/saving a model either from a local
file or directory, or from a pretrained model configuration provided by the library (downloaded from HuggingFace's AWS
S3 repository).

[`PreTrainedModel`] and [`TFPreTrainedModel`] also implement a few methods which
are common among all the models to:

- resize the input token embeddings when new tokens are added to the vocabulary
- prune the attention heads of the model.

The other methods that are common to each model are defined in [`~modeling_utils.ModuleUtilsMixin`]
(for the PyTorch models) and [`~modeling_tf_utils.TFModuleUtilsMixin`] (for the TensorFlow models) or
for text generation, [`~generation_utils.GenerationMixin`] (for the PyTorch models),
[`~generation_tf_utils.TFGenerationMixin`] (for the TensorFlow models) and
[`~generation_flax_utils.FlaxGenerationMixin`] (for the Flax/JAX models).


## PreTrainedModel

[[autodoc]] PreTrainedModel
    - push_to_hub
    - all

<a id='from_pretrained-torch-dtype'></a>

### Large model loading

In Transformers 4.20.0, the [`~PreTrainedModel.from_pretrained`] method has been reworked to accommodate large models using [Accelerate](https://huggingface.co/docs/accelerate/big_modeling). This requires Accelerate >= 0.9.0 and PyTorch >= 1.9.0. Instead of creating the full model, then loading the pretrained weights inside it (which takes twice the size of the model in RAM, one for the randomly initialized model, one for the weights), there is an option to create the model as an empty shell, then only materialize its parameters when the pretrained weights are loaded.

This option can be activated with `low_cpu_mem_usage=True`. The model is first created on the Meta device (with empty weights) and the state dict is then loaded inside it (shard by shard in the case of a sharded checkpoint). This way the maximum RAM used is the full size of the model only.

```py
from transformers import AutoModelForSeq2SeqLM

t0pp = AutoModelForSeq2SeqLM.from_pretrained("bigscience/T0pp", low_cpu_mem_usage=True)
```

Moreover, you can directly place the model on different devices if it doesn't fully fit in RAM (only works for inference for now). With `device_map="auto"`, Accelerate will determine where to put each layer to maximize the use of your fastest devices (GPUs) and offload the rest on the CPU, or even the hard drive if you don't have enough GPU RAM (or CPU RAM). Even if the model is split across several devices, it will run as you would normally expect.

When passing a `device_map`, `low_cpu_mem_usage` is automatically set to `True`, so you don't need to specify it:

```py
from transformers import AutoModelForSeq2SeqLM

t0pp = AutoModelForSeq2SeqLM.from_pretrained("bigscience/T0pp", device_map="auto")
```

You can inspect how the model was split across devices by looking at its `hf_device_map` attribute:

```py
t0pp.hf_device_map
```

```python out
{'shared': 0,
 'decoder.embed_tokens': 0,
 'encoder': 0,
 'decoder.block.0': 0,
 'decoder.block.1': 1,
 'decoder.block.2': 1,
 'decoder.block.3': 1,
 'decoder.block.4': 1,
 'decoder.block.5': 1,
 'decoder.block.6': 1,
 'decoder.block.7': 1,
 'decoder.block.8': 1,
 'decoder.block.9': 1,
 'decoder.block.10': 1,
 'decoder.block.11': 1,
 'decoder.block.12': 1,
 'decoder.block.13': 1,
 'decoder.block.14': 1,
 'decoder.block.15': 1,
 'decoder.block.16': 1,
 'decoder.block.17': 1,
 'decoder.block.18': 1,
 'decoder.block.19': 1,
 'decoder.block.20': 1,
 'decoder.block.21': 1,
 'decoder.block.22': 'cpu',
 'decoder.block.23': 'cpu',
 'decoder.final_layer_norm': 'cpu',
 'decoder.dropout': 'cpu',
 'lm_head': 'cpu'}
```

You can also write your own device map following the same format (a dictionary layer name to device). It should map all parameters of the model to a given device, but you don't have to detail where all the submosules of one layer go if that layer is entirely on the same device. For instance, the following device map would work properly for T0pp (as long as you have the GPU memory):

```python
device_map = {"shared": 0, "encoder": 0, "decoder": 1, "lm_head": 1}
```

Another way to minimize the memory impact of your model is to instantiate it at a lower precision dtype (like `torch.float16`) or use direct quantization techniques as described below.

### Model Instantiation dtype

Under Pytorch a model normally gets instantiated with `torch.float32` format. This can be an issue if one tries to
load a model whose weights are in fp16, since it'd require twice as much memory. To overcome this limitation, you can
either explicitly pass the desired `dtype` using `torch_dtype` argument:

```python
model = T5ForConditionalGeneration.from_pretrained("t5", torch_dtype=torch.float16)
```

or, if you want the model to always load in the most optimal memory pattern, you can use the special value `"auto"`,
and then `dtype` will be automatically derived from the model's weights:

```python
model = T5ForConditionalGeneration.from_pretrained("t5", torch_dtype="auto")
```

Models instantiated from scratch can also be told which `dtype` to use with:

```python
config = T5Config.from_pretrained("t5")
model = AutoModel.from_config(config)
```

Due to Pytorch design, this functionality is only available for floating dtypes.

### `bitsandbytes` integration for Int8 mixed-precision matrix decomposition

From the paper `GPT3.int8() : 8-bit Matrix Multiplication for Transformers at Scale`, we suport HuggingFace 🤗  integration for all models in the Hub with few lines of code. 
For models trained in  half-precision (aka, either `float16` or `bfloat16`) or full precision. This method aims to reduce `nn.Linear` size by 2 (if trained in half precision) or by 4 if trained in full precision, without affecting too much quality by operating on the outliers in half-precision.
This technique is useful and works well for billion scale models (>1B parameters) therefore we advice you to use it only for models of that scale. This method has been tested for 2-billion to 176-billion scale models and supports only PyTorch models. 

![HFxbitsandbytes.png](https://s3.amazonaws.com/moonup/production/uploads/1659861207959-62441d1d9fdefb55a0b7d12c.png)

Int8 mixed-precision matrix decomposition works by separating a matrix multiplication into two streams: (1) and systematic feature outlier stream matrix multiplied in fp16 (0.01%), (2) a regular stream of int8 matrix multiplication (99.9%). With this method, int8 inference with no predictive degradation is possible for very large models (>=176B parameters).
Values are usually normally distributed, that is, most values are in the range [-3.5, 3.5], but there are some exceptional systematic outliers that are very differently distributed for large models. These outliers are often in the interval [-60, -6] or [6, 60]. Int8 quantization works well for values of magnitude ~5, but beyond that, there is a significant performance penalty. A good default threshold is 6, but a lower threshold might be needed for more unstable models (small models, fine-tuning).

Note also that you would require a GPU to run mixed-8bit models as the kernels has been compiled for GPUs only. Make sure that you have enough GPU RAM to store the quarter (or half if your model is natively in half precision) of the model before using this feature. 

Below are some notes to help you use this module, or follow this demo on Google colab: [![Open In Google Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/drive/1qOjXfQIAULfKvZqwCen8-MoWKGdSatZ4?usp=sharing)

#### Requirements

- Make sure you run that on a NVIDIA GPU that supports 8-bit tensor cores (Turing or Ampere GPUs - e.g. T4, RTX20s RTX30s, A40-A100). Note that previous generations of NVIDIA GPUs do not support 8-bit tensor cores.
- Install the correct version of `bitsandbytes` by running:
`pip install -i https://test.pypi.org/simple/ bitsandbytes`
- Install `accelerate`:
`pip install accelerate`

#### Running mixed-int8 models

After carefully installing the required libraries, the way to load your mixed 8-bit model is as follows:
```py
model_name = "bigscience/bloom-2b5"
model_8bit = AutoModelForCausalLM.from_pretrained(model_name, device_map="auto", load_in_8bit=True)
```
The implementation supports multi-GPU setup thanks to `accelerate` as backend. If you want to control the GPU memory you want to allocate for each GPU, you can use the `max_memory` argument as follows:
(If allocating `1GB` into GPU-0 and `2GB` into GPU-1, you can use `max_memory={0:"1GB", 1:"2GB"}`)
```py
max_memory_mapping = {0: "1GB", 1: "2GB"}
model_name = "bigscience/bloom-3b"
model_8bit = AutoModelForCausalLM.from_pretrained(
    model_name, device_map="auto", load_in_8bit=True, max_memory=max_memory_mapping
)
```


## ModuleUtilsMixin

[[autodoc]] modeling_utils.ModuleUtilsMixin

## TFPreTrainedModel

[[autodoc]] TFPreTrainedModel
    - push_to_hub
    - all

## TFModelUtilsMixin

[[autodoc]] modeling_tf_utils.TFModelUtilsMixin

## FlaxPreTrainedModel

[[autodoc]] FlaxPreTrainedModel
    - push_to_hub
    - all

## Pushing to the Hub

[[autodoc]] utils.PushToHubMixin

## Sharded checkpoints

[[autodoc]] modeling_utils.load_sharded_checkpoint