#
torchao

torchao is a PyTorch architecture optimization library with support for custom high performance data types, quantization, and sparsity. It is composable with native PyTorch features such as torch.compile for even faster inference and training.

Install torchao with the following command.

    # Updating 🤗 Transformers to the latest version, as the example script below uses the new auto compilation
    pip install --upgrade torch torchao transformers

torchao supports many quantization types for different data types (int4, float8, weight only, etc.). Starting with version 0.10.0, torchao provides enhanced flexibility through the `AOBaseConfig` API, allowing for more customized quantization configurations. And full access to the techniques offered in the torchao library.

You can manually choose the quantization types and settings or automatically select the quantization types.

manual
automatic

Create a TorchAoConfig and specify the quantization type and `group_size` of the weights to quantize. Set the `cache_implementation` to `"static"` to automatically torch.compile the forward method.

Run the quantized model on a CPU by changing `device_map` to `"cpu"` and `layout` to `Int4CPULayout()`. This is only available in torchao 0.8.0+.

In torchao 0.10.0+, you can use the more flexible `AOBaseConfig` approach instead of string identifiers:

    import torch
    from transformers import TorchAoConfig, AutoModelForCausalLM, AutoTokenizer
    from torchao.quantization import Int4WeightOnlyConfig

    # Using AOBaseConfig instance (torchao >= 0.10.0)
    quant_config = Int4WeightOnlyConfig(group_size=128)
    quantization_config = TorchAoConfig(quant_type=quant_config)

    # Load and quantize the model
    quantized_model = AutoModelForCausalLM.from_pretrained(
        "meta-llama/Meta-Llama-3-8B",
        torch_dtype="auto",
        device_map="auto",
        quantization_config=quantization_config
    )

    tokenizer = AutoTokenizer.from_pretrained("meta-llama/Meta-Llama-3-8B")
    input_text = "What are we having for dinner?"
    input_ids = tokenizer(input_text, return_tensors="pt").to("cuda")

    # auto-compile the quantized model with `cache_implementation="static"` to get speed up
    output = quantized_model.generate(**input_ids, max_new_tokens=10, cache_implementation="static")
    print(tokenizer.decode(output[0], skip_special_tokens=True))

##
Available Quantization Schemes

TorchAO provides a variety of quantization configurations:

- `Int4WeightOnlyConfig`
- `Int8WeightOnlyConfig`
- `Int8DynamicActivationInt8WeightConfig`
- `Float8WeightOnlyConfig`

Each configuration can be further customized with parameters such as `group_size`, `scheme`, and `layout` to optimize for specific hardware and model architectures.

For a complete list of available configurations, see our quantization API documentation.

> ⚠️ DEPRECATION WARNING

> Starting with version 0.10.0, the string-based API for quantization configuration (e.g., `TorchAoConfig("int4_weight_only", group_size=128)`) is deprecated and will be removed in a future release.

> Please use the new `AOBaseConfig`-based approach instead:

>     # Old way (deprecated)
>     quantization_config = TorchAoConfig("int4_weight_only", group_size=128)

>     # New way (recommended)
>     from torchao.quantization import Int4WeightOnlyConfig
>     quant_config = Int4WeightOnlyConfig(group_size=128)
>     quantization_config = TorchAoConfig(quant_type=quant_config)

> The new API offers greater flexibility, better type safety, and access to the full range of features available in torchao.

> ##
> Migration Guide

> Here’s how to migrate from common string identifiers to their `AOBaseConfig` equivalents:

> Old String API | New `AOBaseConfig` API
> --- | ---
> `"int4_weight_only"` | `Int4WeightOnlyConfig()`
> `"int8_weight_only"` | `Int8WeightOnlyConfig()`
> `"int8_dynamic_activation_int8_weight"` | `Int8DynamicActivationInt8WeightConfig()`

> All configuration objects accept parameters for customization (e.g., `group_size`, `scheme`, `layout`).

Below is the API for for torchao < `0.9.0`

    import torch
    from transformers import TorchAoConfig, AutoModelForCausalLM, AutoTokenizer

    quantization_config = TorchAoConfig("int4_weight_only", group_size=128)
    quantized_model = AutoModelForCausalLM.from_pretrained(
        "meta-llama/Meta-Llama-3-8B",
        torch_dtype="auto",
        device_map="auto",
        quantization_config=quantization_config
    )

    tokenizer = AutoTokenizer.from_pretrained("meta-llama/Meta-Llama-3-8B")
    input_text = "What are we having for dinner?"
    input_ids = tokenizer(input_text, return_tensors="pt").to("cuda")

    # auto-compile the quantized model with `cache_implementation="static"` to get speed up
    output = quantized_model.generate(**input_ids, max_new_tokens=10, cache_implementation="static")
    print(tokenizer.decode(output[0], skip_special_tokens=True))

Run the code below to benchmark the quantized models performance.

    from torch._inductor.utils import do_bench_using_profiling
    from typing import Callable

    def benchmark_fn(func: Callable, *args, **kwargs) -> float:
        """Thin wrapper around do_bench_using_profiling"""
        no_args = lambda: func(*args, **kwargs)
        time = do_bench_using_profiling(no_args)
        return time * 1e3

    MAX_NEW_TOKENS = 1000
    print("int4wo-128 model:", benchmark_fn(quantized_model.generate, **input_ids, max_new_tokens=MAX_NEW_TOKENS, cache_implementation="static"))

    bf16_model = AutoModelForCausalLM.from_pretrained(model_name, device_map="auto", torch_dtype=torch.bfloat16)
    output = bf16_model.generate(**input_ids, max_new_tokens=10, cache_implementation="static") # auto-compile
    print("bf16 model:", benchmark_fn(bf16_model.generate, **input_ids, max_new_tokens=MAX_NEW_TOKENS, cache_implementation="static"))

For best performance, you can use recommended settings by calling `torchao.quantization.utils.recommended_inductor_config_setter()`

##
Serialization

torchao implements torch.Tensor subclasses for maximum flexibility in supporting new quantized torch.Tensor formats. Safetensors serialization and deserialization does not work with torchao.

To avoid arbitrary user code execution, torchao sets `weights_only=True` in torch.load to ensure only tensors are loaded. Any known user functions can be whitelisted with add_safe_globals.

    # don't serialize model with Safetensors
    output_dir = "llama3-8b-int4wo-128"
    quantized_model.save_pretrained("llama3-8b-int4wo-128", safe_serialization=False)

##
Resources

For a better sense of expected performance, view the benchmarks for various models with CUDA and XPU backends.

Refer to Other Available Quantization Techniques for more examples and documentation.

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