问题产生的原因是使用nn.CrossEntropyLoss()来计算损失的时候，target的维度超过4
import torch
import torch.nn as nn
logit = torch.ones(size=(4, 32, 256, 256)) # b,c,h,w
target = torch.ones(size=(4, 1, 256, 256))
criterion = nn.CrossEntropyLoss()
loss = criterion(logit, target)
如实target中的C不是1，则可以：
import torch
import torch.nn as nn
logit = torch.ones(size=(4, 32, 256, 256)) # b,c,h,w
target = torch.ones(size=(4, 2, 256, 256))
criterion = nn.CrossEntropyLoss()
losses = 0
for i in range(2):
loss = criterion(logit, target[:, i, ...].long())
losses += loss可以看到代码里面有个.long(),如果不用的话则会报错：
RuntimeError: expected scalar type Long but found Float
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