PyTorch adapt binary classification model to output probabilities of both classes
My dataset has 14 features and a target containing {0,1}.
I have trained this binary classifier:
class SimpleBinaryClassifier(nn.Module):
    def __init__(self,input_shape):
        super().__init__()
        self.fc1 = nn.Linear(input_shape,64)
        self.fc2 = nn.Linear(64,32)
        self.dropout = nn.Dropout(p=0.1)
        self.fc3 = nn.Linear(32,1)
        
    def forward(self,x):
        x = torch.relu(self.fc1(x))
        x = torch.relu(self.fc2(x))
        x = self.dropout(x)
        x = self.fc3(x)
        return x

with the following criterion and training loop:
criterion   = nn.BCEWithLogitsLoss()

def binary_acc(y_pred, y_test):
    y_pred_tag = torch.round(torch.sigmoid(y_pred))
    correct_results_sum = (y_pred_tag == y_test).sum().float()
    acc = correct_results_sum/y_test.shape[0]
    acc = torch.round(acc * 100)
    
    return acc

model.train()
for e in range(1, EPOCHS+1):
    epoch_loss = 0
    epoch_acc = 0
    for X_batch, y_batch in train_loader:
        X_batch, y_batch = X_batch.to(device), y_batch.to(device)
        optimizer.zero_grad()
        
        y_pred = model(X_batch)
        
        loss = criterion(y_pred, y_batch.unsqueeze(1))
        acc = binary_acc(y_pred, y_batch.unsqueeze(1))
        
        loss.backward()
        optimizer.step()
        
        epoch_loss += loss.item()
        epoch_acc += acc.item()
        

    print(f'Epoch {e+0:03}: | Loss: {epoch_loss/len(train_loader):.5f} | Acc: {epoch_acc/len(train_loader):.3f}')

This model, when called like sigmoid(model(input_tensor)) outputs a single number in [0,1]. The pipeline I'm working with, expects a model to output probabilities [p_class1, p_class2].
How can I adapt the model and the training loop?
If I set the output of the last layer to 2, I have problems with the criterion inside the training loop.
class SimpleBinaryClassifier2(nn.Module):
    def __init__(self,input_shape):
        super().__init__()
        self.fc1 = nn.Linear(input_shape,64)
        self.fc2 = nn.Linear(64,32)
        self.dropout = nn.Dropout(p=0.1)
        self.fc3 = nn.Linear(32,2) # now it's 2
        
    def forward(self,x):
        x = torch.relu(self.fc1(x))
        x = torch.relu(self.fc2(x))
        x = self.dropout(x)
        x = self.fc3(x)
        return x

I use the CrossEntropy
model = SimpleBinaryClassifier2(input_shape=14)
model.to(device)
print(model)
optimizer = torch.optim.Adam(model.parameters(), lr=LEARNING_RATE)
criterion   = nn.CrossEntropyLoss()

and replace y_pred_tag = torch.round(torch.sigmoid(y_pred)) with argmax(softmax)
def binary_acc2(y_pred, y_test):
    y_pred_tag = torch.argmax(torch.softmax(y_pred), dim=1)
    correct_results_sum = (y_pred_tag == y_test).sum().float()
    acc = correct_results_sum/y_test.shape[0]
    acc = torch.round(acc * 100)
    return acc

Then the train loop rises an error:
model.train()
for e in range(1, EPOCHS+1):
    epoch_loss = 0
    epoch_acc = 0
    for X_batch, y_batch in train_loader:
        X_batch, y_batch = X_batch.to(device), y_batch.to(device)
        optimizer.zero_grad()
        
        y_pred = model(X_batch)
        
        loss = criterion(y_pred, y_batch)
        acc = binary_acc(y_pred, y_batch)
        
        loss.backward()
        optimizer.step()
        
        epoch_loss += loss.item()
        epoch_acc += acc.item()
        
​
    print(f'Epoch {e+0:03}: | Loss: {epoch_loss/len(train_loader):.5f} | Acc: {epoch_acc/len(train_loader):.3f}')

The error is the following:
RuntimeError: &quot;nll_loss_forward_reduce_cuda_kernel_2d_index&quot; not implemented for 'Float'

I already looked up on this other post where the cause of that error was that the element was a Float and not a tensor, but in my case the datasets are tensors:
train_dataset = GenericDataset(torch.FloatTensor(X_train), torch.FloatTensor(y_train))
test_dataset = GenericDataset(torch.FloatTensor(X_test), torch.FloatTensor(y_test))


According to nn.CrossEntropyLoss description it expects target as long and not float, while in your train_dataset you clearly convert it to float
