Optimum Quanto

🤗 Optimum Quanto is a pytorch quantization backend for optimum.

It has been designed with versatility and simplicity in mind:

• all features are available in eager mode (works with non-traceable models),
• quantized models can be placed on any device (including CUDA and MPS),
• automatically inserts quantization and dequantization stubs,
• automatically inserts quantized functional operations,
• automatically inserts quantized modules (see below the list of supported modules),
• provides a seamless workflow from a float model to a dynamic to a static quantized model,
• serialization compatible with pytorch weight_only and 🤗 safetensors,
• accelerated matrix multiplications on CUDA devices (int8-int8, fp16-int4, bf16-int8, bf16-int4),
• supports int2, int4, int8 and float8 weights,
• supports int8 and float8 activations.

Features yet to be implemented:

• dynamic activations smoothing,
• kernels for all mixed matrix multiplications on all devices,
• compatibility with torch compiler (aka dynamo).

Performances

In a nutshell:

• accuracy: models compiled with int8/float8 weights and float8 activations are very close to the full-precision models,
• latency: whenever optimized kernels are available, the inference of quantized model is comparable with the full-precision models when quantizing only the model weights,
• device memory: approximately divided by float bits / integer bits.

The paragraph below is just an example. Please refer to the bench folder for detailed results per use-case of model.

meta-llama/Meta-Llama-3.1-8B

Installation

Optimum Quanto is available as a pip package.

pip install optimum-quanto

Quantization workflow for Hugging Face models

optimum-quanto provides helper classes to quantize, save and reload Hugging Face quantized models.

LLM models

The first step is to quantize the model

from transformers import AutoModelForCausalLM
from optimum.quanto import QuantizedModelForCausalLM, qint4

model = AutoModelForCausalLM.from_pretrained('meta-llama/Meta-Llama-3-8B')
qmodel = QuantizedModelForCausalLM.quantize(model, weights=qint4, exclude='lm_head')

Note: the model quantized weights will be frozen. If you want to keep them unfrozen to train them you need to use optimum.quanto.quantize directly.

The quantized model can be saved using save_pretrained:

qmodel.save_pretrained('./Llama-3-8B-quantized')

It can later be reloaded using from_pretrained:

from optimum.quanto import QuantizedModelForCausalLM

qmodel = QuantizedModelForCausalLM.from_pretrained('Llama-3-8B-quantized')

Diffusers models

You can quantize any of the submodels inside a diffusers pipeline and seamlessly include them later in another pipeline.

Here we quantize the transformer of a Pixart pipeline.

from diffusers import PixArtTransformer2DModel
from optimum.quanto import QuantizedPixArtTransformer2DModel, qfloat8

model = PixArtTransformer2DModel.from_pretrained("PixArt-alpha/PixArt-Sigma-XL-2-1024-MS", subfolder="transformer")
qmodel = QuantizedPixArtTransformer2DModel.quantize(model, weights=qfloat8)
qmodel.save_pretrained("./pixart-sigma-fp8")

Later, we can reload the quantized model and recreate the pipeline:

from diffusers import PixArtTransformer2DModel
from optimum.quanto import QuantizedPixArtTransformer2DModel

transformer = QuantizedPixArtTransformer2DModel.from_pretrained("./pixart-sigma-fp8")
transformer.to(device="cuda")
pipe = PixArtSigmaPipeline.from_pretrained(
  "PixArt-alpha/PixArt-Sigma-XL-2-1024-MS",
  transformer=None,
  torch_dtype=torch.float16,
).to("cuda")
pipe.transformer = transformer

Quantization workflow for vanilla pytorch models (low-level API)

One thing to keep in mind when using the low-level quanto API is that by default models weights are dynamically quantized: an explicit call must be made to 'freeze' the quantized weights.

A typical quantization workflow would consist of the following steps:

1. Quantize

The first step converts a standard float model into a dynamically quantized model.

from optimum.quanto import quantize, qint8

quantize(model, weights=qint8, activations=qint8)

At this stage, only the inference of the model is modified to dynamically quantize the weights.

2. Calibrate (optional if activations are not quantized)

Quanto supports a calibration mode that allows to record the activation ranges while passing representative samples through the quantized model.

from optimum.quanto import Calibration

with Calibration(momentum=0.9):
    model(samples)

This automatically activates the quantization of the activations in the quantized modules.

3. Tune, aka Quantization-Aware-Training (optional)

If the performance of the model degrades too much, one can tune it for a few epochs to recover the float model performance.

import torch

model.train()
for batch_idx, (data, target) in enumerate(train_loader):
    data, target = data.to(device), target.to(device)
    optimizer.zero_grad()
    output = model(data).dequantize()
    loss = torch.nn.functional.nll_loss(output, target)
    loss.backward()
    optimizer.step()

4. Freeze integer weights

When freezing a model, its float weights are replaced by quantized integer weights.

from optimum.quanto import freeze

freeze(model)

5. Serialize quantized model

Quantized models weights can be serialized to a state_dict, and saved to a file. Both pickle and safetensors (recommended) are supported.

from safetensors.torch import save_file

save_file(model.state_dict(), 'model.safetensors')

In order to be able to reload these weights, you also need to store the quantized model quantization map.

import json

from optimum.quanto import quantization_map

with open('quantization_map.json', w) as f:
  json.dump(quantization_map(model))

5. Reload a quantized model

A serialized quantized model can be reloaded from a state_dict and a quantization_map using the requantize helper. Note that you need first to instantiate an empty model.

import json

from safetensors.torch import load_file

state_dict = load_file('model.safetensors')
with open('quantization_map.json', r) as f:
  quantization_map = json.load(f)

# Create an empty model from your modeling code and requantize it
with torch.device('meta'):
  new_model = ...
requantize(new_model, state_dict, quantization_map, device=torch.device('cuda'))

Please refer to the examples for instantiations of that workflow.

Design overview

Tensors

At the heart of quanto is a Tensor subclass that corresponds to:

• the projection of a source Tensor into the optimal range for a given destination type,
• the mapping of projected values to the destination type.

For floating-point destination types, the mapping is done by the native pytorch cast (i.e. Tensor.to()).

For integer destination types, the mapping is a simple rounding operation (i.e. torch.round()).

The goal of the projection is to increase the accuracy of the conversion by minimizing the number of:

• saturated values (i.e. mapped to the destination type min/max),
• zeroed values (because they are below the smallest number that can be represented by the destination type)

The projection is symmetric per-tensor or per-channel for int8 and float8, and group-wise affine (with a shift or 'zero-point') for lower bitwidth.

One of the benefits of using a lower-bitwidth representation is that you will be able to take advantage of accelerated operations for the destination type, which is typically faster than their higher precision equivalents.

Quanto does not support the conversion of a Tensor using mixed destination types.

Modules

Quanto provides a generic mechanism to replace torch modules by optimum-quanto modules that are able to process quanto tensors.

optimum-quanto modules dynamically convert their weights until a model is frozen, which slows down inference a bit but is required if the model needs to be tuned.

Weights are usually quantized per-channel along the first dimension (output features).

Biases are not converted to preserve the accuracy of a typical addmm operation.

Explanation: to be consistent with the unquantized arithmetic operations, biases would need to be quantized with a scale that is equal to the product of the input and weight scales, which leads to a ridiculously small scale, and conversely requires a very high bitwidth to avoid clipping. Typically, with int8 inputs and weights, biases would need to be quantized with at least 12 bits, i.e. in int16. Since most biases are today float16, this is a waste of time.

Activations are dynamically quantized per-tensor using static scales (defaults to the range [-1, 1]).

To preserve accuracy, the model needs to be calibrated to evaluate the best activation scales (using a momentum).

The following modules can be quantized:

• Linear (QLinear). Weights are always quantized, and biases are not quantized. Inputs and outputs can be quantized.
• Conv2d (QConv2D). Weights are always quantized, and biases are not quantized. Inputs and outputs can be quantized.
• LayerNorm, Weights and biases are not quantized. Outputs can be quantized.

Pitfalls to avoid when quantizing activations

Activations are always quantized per-tensor because most linear algebra operations in a model graph are not compatible with per-axis inputs: you simply cannot add numbers that are not expressed in the same base (you cannot add apples and oranges).

Weights involved in matrix multiplications are, on the contrary, always quantized along their first axis, because all output features are evaluated independently from one another.

The outputs of a quantized matrix multiplication will anyway always be dequantized, even if activations are quantized, because:

• the resulting accumulated values are expressed with a much higher bitwidth (typically int32 or float32) than the activation bitwidth (typically int8 or float8),
• they might be combined with a float bias.

Quantizing activations per-tensor to int8 can lead to serious quantization errors if the corresponding tensors contain large outlier values. Typically, this will lead to quantized tensors with most values set to zero (except the outliers).

A possible solution to work around that issue is to 'smooth' the activations statically as illustrated by SmoothQuant. You can find a script to smooth some model architectures under external/smoothquant.

A better option is to represent activations using float8.