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# Hybrid Vision Transformer (ViT Hybrid)

## Overview

The hybrid Vision Transformer (ViT) model was proposed in [An Image is Worth 16x16 Words: Transformers for Image Recognition
at Scale](https://arxiv.org/abs/2010.11929) by Alexey Dosovitskiy, Lucas Beyer, Alexander Kolesnikov, Dirk
Weissenborn, Xiaohua Zhai, Thomas Unterthiner, Mostafa Dehghani, Matthias Minderer, Georg Heigold, Sylvain Gelly, Jakob
Uszkoreit, Neil Houlsby. It's the first paper that successfully trains a Transformer encoder on ImageNet, attaining
very good results compared to familiar convolutional architectures. ViT hybrid is a slight variant of the [plain Vision Transformer](vit),
by leveraging a convolutional backbone (specifically, [BiT](bit)) whose features are used as initial "tokens" for the Transformer.

The abstract from the paper is the following:

*While the Transformer architecture has become the de-facto standard for natural language processing tasks, its
applications to computer vision remain limited. In vision, attention is either applied in conjunction with
convolutional networks, or used to replace certain components of convolutional networks while keeping their overall
structure in place. We show that this reliance on CNNs is not necessary and a pure transformer applied directly to
sequences of image patches can perform very well on image classification tasks. When pre-trained on large amounts of
data and transferred to multiple mid-sized or small image recognition benchmarks (ImageNet, CIFAR-100, VTAB, etc.),
Vision Transformer (ViT) attains excellent results compared to state-of-the-art convolutional networks while requiring
substantially fewer computational resources to train.*

This model was contributed by [nielsr](https://huggingface.co/nielsr). The original code (written in JAX) can be
found [here](https://github.com/google-research/vision_transformer).

## Using Scaled Dot Product Attention (SDPA)

PyTorch includes a native scaled dot-product attention (SDPA) operator as part of `torch.nn.functional`. This function 
encompasses several implementations that can be applied depending on the inputs and the hardware in use. See the 
[official documentation](https://pytorch.org/docs/stable/generated/torch.nn.functional.scaled_dot_product_attention.html) 
or the [GPU Inference](https://huggingface.co/docs/transformers/main/en/perf_infer_gpu_one#pytorch-scaled-dot-product-attention)
page for more information.

SDPA is used by default for `torch>=2.1.1` when an implementation is available, but you may also set 
`attn_implementation="sdpa"` in `from_pretrained()` to explicitly request SDPA to be used.

```
from transformers import ViTHybridForImageClassification
model = ViTHybridForImageClassification.from_pretrained("google/vit-hybrid-base-bit-384", attn_implementation="sdpa", torch_dtype=torch.float16)
...
```

For the best speedups, we recommend loading the model in half-precision (e.g. `torch.float16` or `torch.bfloat16`).

On a local benchmark (A100-40GB, PyTorch 2.3.0, OS Ubuntu 22.04) with `float32` and `google/vit-hybrid-base-bit-384` model, we saw the following speedups during inference.

|   Batch size |   Average inference time (ms), eager mode |   Average inference time (ms), sdpa model |   Speed up, Sdpa / Eager (x) |
|--------------|-------------------------------------------|-------------------------------------------|------------------------------|
|            1 |                                        29 |                                        18 |                      1.61 |
|            2 |                                        26 |                                        18 |                      1.44 |
|            4 |                                        25 |                                        18 |                      1.39 |
|            8 |                                        34 |                                        24 |                      1.42 |

## Resources

A list of official Hugging Face and community (indicated by 🌎) resources to help you get started with ViT Hybrid.

<PipelineTag pipeline="image-classification"/>

- [`ViTHybridForImageClassification`] is supported by this [example script](https://github.com/huggingface/transformers/tree/main/examples/pytorch/image-classification) and [notebook](https://colab.research.google.com/github/huggingface/notebooks/blob/main/examples/image_classification.ipynb).
- See also: [Image classification task guide](../tasks/image_classification)

If you're interested in submitting a resource to be included here, please feel free to open a Pull Request and we'll review it! The resource should ideally demonstrate something new instead of duplicating an existing resource.

## ViTHybridConfig

[[autodoc]] ViTHybridConfig

## ViTHybridImageProcessor

[[autodoc]] ViTHybridImageProcessor
    - preprocess

## ViTHybridModel

[[autodoc]] ViTHybridModel
    - forward

## ViTHybridForImageClassification

[[autodoc]] ViTHybridForImageClassification
    - forward
