acc = (logits.argmax(dim=-1) == labels).float().mean()什么意思

这行代码计算了模型的准确率（accuracy），即预测正确的样本数占总样本数的比例。具体来说，它首先通过 `logits.argmax(dim=-1)` 获取模型在每个样本上预测的类别（即概率最大的类别），然后与标签进行比较，得到一个布尔值张量，其中预测正确的位置为 True，错误的位置为 False。最后，将这个张量转化为浮点数并取平均值，即可得到准确率。

相关问题

Structure-Aware Transformer for Graph Representation Learning 论文代码的 dgl实现

"Structure-Aware Transformer for Graph Representation Learning"是一篇使用Transformer模型进行图表示学习的论文。这篇论文提出了一种名为SAT（Structure-Aware Transformer）的模型，它利用了图中节点之间的结构信息，以及节点自身的特征信息。SAT模型在多个图数据集上都取得了非常好的结果。

以下是SAT模型的dgl实现代码，代码中使用了Cora数据集进行示例：

import dgl
import numpy as np
import torch
import torch.nn as nn
import torch.nn.functional as F

class GraphAttentionLayer(nn.Module):
    def __init__(self, in_dim, out_dim, num_heads):
        super(GraphAttentionLayer, self).__init__()

        self.num_heads = num_heads
        self.out_dim = out_dim

        self.W = nn.Linear(in_dim, out_dim*num_heads, bias=False)
        nn.init.xavier_uniform_(self.W.weight)

        self.a = nn.Parameter(torch.zeros(size=(2*out_dim, 1)))
        nn.init.xavier_uniform_(self.a.data)

    def forward(self, g, h):
        h = self.W(h).view(-1, self.num_heads, self.out_dim)

        # Compute attention scores
        with g.local_scope():
            g.ndata['h'] = h
            g.apply_edges(fn.u_dot_v('h', 'h', 'e'))
            e = F.leaky_relu(g.edata.pop('e'), negative_slope=0.2)
            g.edata['a'] = torch.cat([e, e], dim=1)
            g.edata['a'] = torch.matmul(g.edata['a'], self.a).squeeze()
            g.edata['a'] = F.leaky_relu(g.edata['a'], negative_slope=0.2)
            g.apply_edges(fn.e_softmax('a', 'w'))

        # Compute output features
        g.ndata['h'] = h
        g.update_all(fn.u_mul_e('h', 'w', 'm'), fn.sum('m', 'h'))
        h = g.ndata['h']

        return h.view(-1, self.num_heads*self.out_dim)

class SATLayer(nn.Module):
    def __init__(self, in_dim, out_dim, num_heads):
        super(SATLayer, self).__init__()

        self.attention = GraphAttentionLayer(in_dim, out_dim, num_heads)
        self.dropout = nn.Dropout(0.5)
        self.norm = nn.LayerNorm(out_dim*num_heads)

    def forward(self, g, h):
        h = self.attention(g, h)
        h = self.norm(h)
        h = F.relu(h)
        h = self.dropout(h)
        return h

class SAT(nn.Module):
    def __init__(self, in_dim, hidden_dim, out_dim, num_heads):
        super(SAT, self).__init__()

        self.layer1 = SATLayer(in_dim, hidden_dim, num_heads)
        self.layer2 = SATLayer(hidden_dim*num_heads, out_dim, 1)

    def forward(self, g, h):
        h = self.layer1(g, h)
        h = self.layer2(g, h)
        return h.mean(0)

# Load Cora dataset
from dgl.data import citation_graph as citegrh

data = citegrh.load_cora()
g = data.graph
features = torch.FloatTensor(data.features)
labels = torch.LongTensor(data.labels)
train_mask = torch.BoolTensor(data.train_mask)
val_mask = torch.BoolTensor(data.val_mask)
test_mask = torch.BoolTensor(data.test_mask)

# Add self loop
g = dgl.remove_self_loop(g)
g = dgl.add_self_loop(g)

# Define model and optimizer
model = SAT(features.shape[1], 64, data.num_classes, 8)
optimizer = torch.optim.Adam(model.parameters(), lr=0.005, weight_decay=5e-4)

# Train model
for epoch in range(200):
    model.train()

    logits = model(g, features)
    loss = F.cross_entropy(logits[train_mask], labels[train_mask])

    optimizer.zero_grad()
    loss.backward()
    optimizer.step()

    acc = (logits[val_mask].argmax(1) == labels[val_mask]).float().mean()

    if epoch % 10 == 0:
        print('Epoch {:03d} | Loss {:.4f} | Accuracy {:.4f}'.format(epoch, loss.item(), acc.item()))

# Test model
model.eval()
logits = model(g, features)
acc = (logits[test_mask].argmax(1) == labels[test_mask]).float().mean()
print('Test accuracy {:.4f}'.format(acc.item()))

在这个示例中，我们首先加载了Cora数据集，并将其转换为一个DGL图。然后，我们定义了一个包含两个SAT层的模型，以及Adam优化器。在训练过程中，我们使用交叉熵损失函数和验证集上的准确率来监控模型的性能。在测试阶段，我们计算测试集上的准确率。