Multi class classification - RuntimeError: 1D target tensor expected, multi-target not supported
My goal is to build a multi-class image classifier using Pytorch and based on the EMNIST dataset (black and white pictures of letters).
The shape of my training data X_train is (124800, 28, 28).
The shape of the original target variables y_train is (124800, 1), however I created a one-hot encoding so that now the shape is (124800, 26).
The model that I am building should have 26 output variables, each representing the probability of one letter.
I read in my data as follows:
import scipy .io
emnist = scipy.io.loadmat(DATA_DIR + '/emnist-letters.mat')
data = emnist ['dataset']
X_train = data ['train'][0, 0]['images'][0, 0]
X_train = X_train.reshape((-1,28,28), order='F')

y_train = data ['train'][0, 0]['labels'][0, 0]

Then, I created a one-hot-encoding as follows:
y_train_one_hot = np.zeros([len(y_train), 27])

for i in range (0, len(y_train)):
    y_train_one_hot[i, y_train[i][0]] = 1
    
y_train_one_hot = np.delete(y_train_one_hot, 0, 1)

I create the dataset with:
train_dataset = torch.utils.data.TensorDataset(torch.from_numpy(X_train), torch.from_numpy(y_train_one_hot))

batch_size = 128
n_iters = 3000
num_epochs = n_iters / (len(train_dataset) / batch_size)
num_epochs = int(num_epochs)

train_loader = torch.utils.data.DataLoader(dataset=train_dataset, 
                                           batch_size=batch_size, 
                                           shuffle=True)

And then I build my model as follows:
class CNNModel(nn.Module):
    
    def __init__(self):
        super(CNNModel, self).__init__()
        
        # Convolution 1
        self.cnn1 = nn.Conv2d(in_channels=1, out_channels=16, kernel_size=5, stride=1, padding=0)
        self.relu1 = nn.ReLU()
        
        # Max pool 1
        self.maxpool1 = nn.MaxPool2d(2,2)
     
        # Convolution 2
        self.cnn2 = nn.Conv2d(in_channels=16, out_channels=32, kernel_size=5, stride=1, padding=0)
        self.relu2 = nn.ReLU()
        
        # Max pool 2
        self.maxpool2 = nn.MaxPool2d(kernel_size=2)
        
        # Fully connected 1 (readout)
        self.fc1 = nn.Linear(32 * 4 * 4, 26) 

    def forward(self, x):
        # Convolution 1
        out = self.cnn1(x.float())
        out = self.relu1(out)
        
        # Max pool 1
        out = self.maxpool1(out)
        
        # Convolution 2 
        out = self.cnn2(out)
        out = self.relu2(out)
        
        # Max pool 2 
        out = self.maxpool2(out)
        
        # Resize
        # Original size: (100, 32, 7, 7)
        # out.size(0): 100
        # New out size: (100, 32*7*7)
        out = out.view(out.size(0), -1)

        # Linear function (readout)
        out = self.fc1(out)
        
        return out

model = CNNModel()

criterion = nn.CrossEntropyLoss()

learning_rate = 0.01

optimizer = torch.optim.SGD(model.parameters(), lr = learning_rate)

And then I train the model as follows:
iter = 0
for epoch in range(num_epochs):
    for i, (images, labels) in enumerate(train_loader):
        
        # Add a single channel dimension
        # From: [batch_size, height, width]
        # To: [batch_size, 1, height, width]
        images = images.unsqueeze(1)

        # Forward pass to get output/logits
        outputs = model(images)
        
        # Clear gradients w.r.t. parameters
        optimizer.zero_grad()
        
        # Forward pass to get output/logits
        outputs = model(images)

        # Calculate Loss: softmax --&gt; cross entropy loss
        loss = criterion(outputs, labels)
        
        # Getting gradients w.r.t. parameters
        loss.backward()
        
        # Updating parameters
        optimizer.step()
        
        iter += 1
        
        if iter % 500 == 0:
            # Calculate Accuracy         
            correct = 0
            total = 0
            # Iterate through test dataset
            for images, labels in test_loader:
               
                images = images.unsqueeze(1)
                
                # Forward pass only to get logits/output
                outputs = model(images)
                
                # Get predictions from the maximum value
                _, predicted = torch.max(outputs.data, 1)
                
                # Total number of labels
                total += labels.size(0)
                
                correct += (predicted == labels).sum()
            
            accuracy = 100 * correct / total
            
            # Print Loss
            print('Iteration: {}. Loss: {}. Accuracy: {}'.format(iter, loss.data[0], accuracy))

However, when I run this, I get the following error:
    ---------------------------------------------------------------------------
RuntimeError                              Traceback (most recent call last)
&lt;ipython-input-11-c26c43bbc32e&gt; in &lt;module&gt;()
     21 
     22         # Calculate Loss: softmax --&gt; cross entropy loss
---&gt; 23         loss = criterion(outputs, labels)
     24 
     25         # Getting gradients w.r.t. parameters

3 frames
/usr/local/lib/python3.6/dist-packages/torch/nn/modules/module.py in __call__(self, *input, **kwargs)
    548             result = self._slow_forward(*input, **kwargs)
    549         else:
--&gt; 550             result = self.forward(*input, **kwargs)
    551         for hook in self._forward_hooks.values():
    552             hook_result = hook(self, input, result)

/usr/local/lib/python3.6/dist-packages/torch/nn/modules/loss.py in forward(self, input, target)
    930     def forward(self, input, target):
    931         return F.cross_entropy(input, target, weight=self.weight,
--&gt; 932                                ignore_index=self.ignore_index, reduction=self.reduction)
    933 
    934 

/usr/local/lib/python3.6/dist-packages/torch/nn/functional.py in cross_entropy(input, target, weight, size_average, ignore_index, reduce, reduction)
   2315     if size_average is not None or reduce is not None:
   2316         reduction = _Reduction.legacy_get_string(size_average, reduce)
-&gt; 2317     return nll_loss(log_softmax(input, 1), target, weight, None, ignore_index, None, reduction)
   2318 
   2319 

/usr/local/lib/python3.6/dist-packages/torch/nn/functional.py in nll_loss(input, target, weight, size_average, ignore_index, reduce, reduction)
   2113                          .format(input.size(0), target.size(0)))
   2114     if dim == 2:
-&gt; 2115         ret = torch._C._nn.nll_loss(input, target, weight, _Reduction.get_enum(reduction), ignore_index)
   2116     elif dim == 4:
   2117         ret = torch._C._nn.nll_loss2d(input, target, weight, _Reduction.get_enum(reduction), ignore_index)

RuntimeError: 1D target tensor expected, multi-target not supported

I expect that I do something wrong when I initialize/use my loss function. What can I do so that I can start training my model?

If you are using crossentropy loss you shouldn't one-hot encode your target variable y.
Pytorch crossentropy expects just the class indices as target not their one-hot encoded version.
To cite the doc https://pytorch.org/docs/master/generated/torch.nn.CrossEntropyLoss.html  :
This criterion expects a class index in the range [0, C-1] as the target for each value of a 1D tensor of size minibatch;
