解决pytorch CrossEntropyLoss报错RuntimeError: 1D target tensor expected, multi-target not supported

解决方法

CrossEntropyLoss(预测值，label)需要的输入维度是：

1. 有batch时，预测值维度为2，size为[ batch_size, n ]时，label的维度是1，size为[ batch_size ]
2. 没有batch时，预测值的维度为2，size为[ m, n ]，label的维度是1，size为[ m ]

问题解析

一个案例即可说明：

import torch
import torch.nn as nn
import numpy as np

a = torch.tensor(np.random.random((30, 5)))
b = torch.tensor(np.random.randint(0, 4, (30))).long()
loss = nn.CrossEntropyLoss()

print("a的维度：", a.size()) # torch.Size([30, 5])
print("b的维度：", b.size()) # torch.Size([30])
print(loss(a, b)) # tensor(1.6319, dtype=torch.float64)