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具体的分类效果可以看：【Arxiv 2023】Diffusion Models Beat GANs on Image Classification

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1、线性分类器 (Linear, A)

使用一个简单的线性层，通常与一个激活函数结合使用。

import torch.nn as nnclass LinearClassifier(nn.Module):def __init__(self, input_size, num_classes):super(LinearClassifier, self).__init__()self.linear = nn.Linear(input_size, num_classes)def forward(self, x):return self.linear(x)

2、多层感知机 (Multi-Layer Perceptron, B)

包括多个线性层，每层之间可能有激活函数和dropout层。

class MLPClassifier(nn.Module):def __init__(self, input_size, hidden_size, num_classes):super(MLPClassifier, self).__init__()self.fc1 = nn.Linear(input_size, hidden_size)self.relu = nn.ReLU()self.fc2 = nn.Linear(hidden_size, num_classes)def forward(self, x):x = self.relu(self.fc1(x))x = self.fc2(x)return x

3、卷积神经网络 (Convolutional Neural Network, CNN, C)

使用一系列卷积层，通常包括池化层和全连接层。

class CNNClassifier(nn.Module):def __init__(self, num_classes):super(CNNClassifier, self).__init__()self.conv1 = nn.Conv2d(in_channels=3, out_channels=32, kernel_size=3, stride=1, padding=1)self.pool = nn.MaxPool2d(kernel_size=2, stride=2, padding=0)self.conv2 = nn.Conv2d(32, 64, 3, 1, 1)self.fc = nn.Linear(64 * 7 * 7, num_classes)  # Assuming input size is 28x28def forward(self, x):x = self.pool(F.relu(self.conv1(x)))x = self.pool(F.relu(self.conv2(x)))x = x.view(x.size(0), -1)  # Flatten the tensorx = self.fc(x)return x

4、基于注意力机制的头部 (Attention-Based Heads, D)

使用注意力机制，如Transformer的头部结构。

from torch.nn import TransformerEncoder, TransformerEncoderLayerclass AttentionClassifier(nn.Module):def __init__(self, input_size, num_classes, nhead, nhid, nlayers):super(AttentionClassifier, self).__init__()self.model_type = 'Transformer'self.encoder_layer = TransformerEncoderLayer(d_model=input_size, nhead=nhead, dim_feedforward=nhid)self.transformer_encoder = TransformerEncoder(self.encoder_layer, num_layers=nlayers)self.decoder = nn.Linear(input_size, num_classes)def forward(self, src):output = self.transformer_encoder(src)output = self.decoder(output.mean(dim=1))return output