图片速览 BitNet: 1-bit LLM

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输入数据

• 模型使用absmax 量化方法进行b比特量化,将输入量化到$\left[-Q_b,Q_b\right](Q_b=2^{b-1})$
  $$\begin{gathered} \tilde{x}=\mathrm{Quant}(x)=\mathrm{Clip}(x\times \frac{Q_b}{\gamma },-Q_b+ϵ,Q_b-ϵ), \\ \mathrm{Clip}(x,a,b)=\max (a,\min (b,x)),\gamma =||x|{|}_{\infty }, \end{gathered}$$

• 其中 ε 是一个小的浮点数，可防止在执行截断时溢出。

// https://github.com/kyegomez/BitNet/blob/main/bitnet/bitbnet_b158.py
def absmean_quantize_weights(weights):"""Quantizes the weights to -1, 0, or +1 using an absmean quantization function.Parameters:- weights (Tensor): The weights of a neural network layer.Returns:- Tensor: The quantized weights."""# Calculate the average absolute value (γ) of the weightsgamma = torch.mean(torch.abs(weights))# Scale weights by γ and round to the nearest integer among {-1, 0, +1}quantized_weights = torch.clamp(torch.round(weights / gamma), min=-1, max=1)return quantized_weights

权重

• 权重 W 的二值化可以公式化为：

$$\begin{gathered} \alpha =\frac{1}{nm}\sum _{ij}{W}_{ij} \\ \tilde{W}=\mathrm{Sign}(W-\alpha ), \\ \mathrm{Sign}(W_{ij})=\{\begin{array}{ll}+1,& \text{if}{W}_{ij}>0,\\ -1,& \text{if}{W}_{ij}\le 0,\end{array} \end{gathered}$$

矩阵乘法

• 使用上述量化方程，矩阵乘法可以写成：

$$y=\tilde{W}\tilde{x}$$

• 为了保持量化后的方差，我们在激活量化之前引入了一个 LayerNorm函数。这样，输出 y 的方差就估计为 1

$$y=\tilde{W}\tilde{x}=\tilde{W}\text{Quant}(\text{LN}(x))\times \frac{\beta \gamma }{Q_b}$$
$$\mathrm{LN}(x)=\frac{x-E(x)}{\sqrt{\mathrm{Var}(x)+ϵ}},\beta =\frac{1}{nm}\Vert W{\Vert }_1$$

// https://github.com/kyegomez/BitNet/blob/main/bitnet/bitlinear.py
import torch
from torch import Tensor, nnclass BitLinear(nn.Linear):"""BitLinear is a custom linear layer that performs binarization of weights and quantization of activationsin a group-wise manner.Args:in_features (int): Number of input features.out_features (int): Number of output features.bias (bool, optional): If set to False, the layer will not learn an additive bias. Default is True.num_groups (int, optional): Number of groups to divide the weights and activations into. Default is 1."""def __init__(self,in_features: int,out_features: int,bias: bool = True,num_groups: int = 1,b: int = 8,):super().__init__(in_features, out_features, bias)self.in_features = in_featuresself.out_features = out_featuresself.b = bself.num_groups = num_groupsself.eps = 1e-5self.norm = nn.LayerNorm(in_features)def ste(self, x):"""Applies the sign function for binarization and uses Straight-Through Estimator (STE) during backward pass.Args:x (Tensor): Input tensor.Returns:Tensor: Binarized tensor."""binarized_x = torch.sign(x)binarized_x = (binarized_x - x).detach() + xreturn binarized_xdef binarize_weights_groupwise(self):"""Binarizes the weights of the layer in a group-wise manner using STE.Returns:Tensor: Binarized weights tensor."""group_size = self.weight.shape[0] // self.num_groupsbinarized_weights = torch.zeros_like(self.weight)for g in range(self.num_groups):start_idx = g * group_sizeend_idx = (g + 1) * group_sizeweight_group = self.weight[start_idx:end_idx]alpha_g = weight_group.mean()binarized_weights[start_idx:end_idx] = self.ste(weight_group - alpha_g)return binarized_weightsdef quantize_activations_groupwise(self, x):"""Quantizes the activations of the layer in a group-wise manner.Args:x (Tensor): Input tensor.b (int, optional): Number of bits for quantization. Default is 8.Returns:Tensor: Quantized activations tensor."""Q_b = 2 ** (self.b - 1)group_size = x.shape[0] // self.num_groupsquantized_x = torch.zeros_like(x)for g in range(self.num_groups):start_idx = g * group_sizeend_idx = (g + 1) * group_sizeactivation_group = x[start_idx:end_idx]gamma_g = activation_group.abs().max()quantized_x[start_idx:end_idx] = torch.clamp(activation_group * Q_b / (gamma_g + self.eps),-Q_b + self.eps,Q_b - self.eps,)return quantized_xdef dequantize_activations_groupwise(self, x):"""Dequantizes the activations of the layer in a group-wise manner.Args:x (Tensor): Quantized input tensor.b (int, optional): Number of bits used during the quantization. Default is 8.Returns:Tensor: Dequantized activations tensor."""Q_b = 2 ** (self.b - 1)dequantized_x = torch.zeros_like(x)for g in range(self.num_groups):start_idx = g * x.shape[0] // self.num_groupsend_idx = (g + 1) * x.shape[0] // self.num_groupsquantized_group = x[start_idx:end_idx]gamma_g = quantized_group.abs().max()dequantized_x[start_idx:end_idx] = quantized_group * gamma_g / Q_breturn dequantized_xdef forward(self, x: Tensor) -> Tensor:"""Forward pass of the BitLinear layer.Args:x (Tensor): Input tensor.Returns:Tensor: Output tensor."""# Normalize inputx = self.norm(x)# Binarize weights and quantize activationsbinarized_weights = self.binarize_weights_groupwise()# Perform linear transformationoutput = torch.nn.functional.linear(x, binarized_weights, self.bias)# Quantize activationsoutput = self.quantize_activations_groupwise(output)# Dequantize activationsoutput = self.dequantize_activations_groupwise(output)# Return outputreturn output# Example usage
bitlinear = BitLinear(10, 5, num_groups=2, b=8)
input_tensor = torch.randn(5, 10)  # Example input tensor
output = bitlinear(input_tensor)
print(output)  # Example output tensor

CG

• 【自然语言处理】【大模型】BitNet：用1-bit Transformer训练LLM

• BitNet: Scaling 1-bit Transformers for Large Language Models

• The Era of 1-bit LLMs: All Large Language Models are in 1.58 Bits

• Implementation of “BitNet: Scaling 1-bit Transformers for Large Language Models” in pytorch

• DB-LLM: Accurate Dual-Binarization for Efficient LLMs

• 如何看待微软提出的BitNet b1.58？

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