RuntimeError: Given groups=1, weight of size [16, 1, 3, 3], expected input[16, 3, 1, 28] to have 1 channels, but got 3 channels instead
I know my images have only 1 channel so the first conv layer is (1,16,3,1) , but I have no idea why I got such an error.
Here is my code (I post only the related part).
    org_x = train_csv.drop(['id', 'digit', 'letter'], axis=1).values
    org_x = org_x.reshape(-1, 28, 28, 1)  
    org_x = org_x/255
    org_x = np.array(org_x)
    org_x = org_x.reshape(-1, 1, 28, 28)
    org_x = torch.Tensor(org_x).float()

    x_test = test_csv.drop(['id','letter'], axis=1).values
    x_test = x_test.reshape(-1, 28, 28, 1)     
    x_test = x_test/255
    x_test = np.array(x_test)
    x_test = x_test.reshape(-1, 1, 28, 28)
    x_test = torch.Tensor(x_test).float()

    y = train_csv['digit']
    y = list(y)
    print(len(y))
    org_y = np.zeros([len(y), 1])
    for i in range(len(y)):
        org_y[i] = y[i]
    org_y = np.array(org_y)  
    org_y = torch.Tensor(org_y).float()

    from sklearn.model_selection import train_test_split
    x_train, x_valid, y_train, y_valid = train_test_split(
        org_x, org_y, test_size=0.2, random_state=42)  

I checked the x_train shape is [1638, 1, 28, 28] and the x_valid shape is [410, 1, 28, 28].
    transform = transforms.Compose([transforms.ToPILImage(),
                            transforms.ToTensor(),
                            transforms.Normalize((0.5, ), (0.5, )) ]) 

    
    class kmnistDataset(data.Dataset):
        def __init__(self, images, labels, transforms=None):
            self.x = images
            self.y = labels
            self.transforms = transforms
     
        def __len__(self):
            return (len(self.x))

        def __getitem__(self, idx):
            data = np.asarray(self.x[idx][0:]).astype(np.uint8)
    
            if self.transforms:
                data = self.transforms(data)
        
            if self.y is not None:
                return (data, self.y[idx])
            else:
                return data
    
    train_data = kmnistDataset(x_train, y_train, transforms=transform)
    valid_data = kmnistDataset(x_valid, y_valid, transforms=transform)

    # dataloaders
    train_loader = DataLoader(train_data, batch_size=16, shuffle=True)
    valid_loader = DataLoader(valid_data, batch_size=16, shuffle = False) 

And here is my model
    class Net(nn.Module):
      def __init__(self):
            super(Net, self).__init__()

            self.conv1 = nn.Conv2d(1, 16, 3, padding=1)
            self.conv2 = nn.Conv2d(16, 32, 3, padding=1)
            self.conv3 = nn.Conv2d(32, 64, 3, padding=1)
   
            self.bn1 = nn.BatchNorm2d(16)
            self.pool = nn.MaxPool2d(2, 2)

            unit = 64 * 14 * 14 
            self.fc1 = nn.Linear(unit, 500)
            self.fc2 = nn.Linear(500, 10)
    
        def forward(self, x):
            x = self.pool(F.relu(self.bn1(self.conv1(x))))
            x = F.relu(self.conv2(x))
            x = F.relu(self.conv3(x))
            x = x.view(-1, 128 * 28 * 28)
            x = F.relu(self.fc1(x))
            x = self.fc2(x)
            return x
    

    model = Net()
    print(model)

Lastly,
    n_epochs = 30

    valid_loss_min = np.Inf

    for epoch in range(1, n_epochs+1):
        train_loss = 0
        valid_loss = 0

        ###################
        # train the model #
        ###################
        model.train()
        for data in train_loader:
            inputs, labels = data[0], data[1]
            optimizer.zero_grad()
            output = model(inputs)
            loss = criterion(output, labels)
            loss.backward()
            optimizer.step()
            train_loss += loss.item()*data.size(0)
        
        #####################
        # validate the model#
        #####################
        model.eval()
        for data in valid_loader:
            inputs, labels = data[0], data[1]
            output = model(inputs)
            loss = criterion(output, labels)
            valid_loss += loss.item()*data.size(0)
    
    
        train_loss = train_loss/ len(train_loader.dataset)
        valid_loss = valid_loss / len(valid_loader.dataset)

        print('Epoch: {} \tTraining Loss: {:.6f} \tValidation Loss: {:.6f}'.format(
            epoch, train_loss, valid_loss))

When I run it, I got this error message
RuntimeError: Given groups=1, weight of size [16, 1, 3, 3], expected input[16, 3, 1, 28] to have 1 channels, but got 3 channels instead
To be specific,
    ---------------------------------------------------------------------------
    RuntimeError                              Traceback (most recent call last)
    &lt;ipython-input-14-b8783819421f&gt; in &lt;module&gt;
         14         inputs, labels = data[0], data[1]
         15         optimizer.zero_grad()
    ---&gt; 16         output = model(inputs)
         17         loss = criterion(output, labels)
         18         loss.backward()

    /opt/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py in   _call_impl(self, *input, **kwargs)
        725             result = self._slow_forward(*input, **kwargs)
        726         else:
    --&gt; 727             result = self.forward(*input, **kwargs)
        728         for hook in itertools.chain(
        729                 _global_forward_hooks.values(),

    &lt;ipython-input-12-500e34c49306&gt; in forward(self, x)
         26 
         27     def forward(self, x):
    ---&gt; 28         x = self.pool(F.relu(self.bn1(self.conv1(x))))
         29         x = F.relu(self.conv2(x))
         30         x = F.relu(self.conv3(x))

    /opt/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py in         _call_impl(self, *input, **kwargs)
        725             result = self._slow_forward(*input, **kwargs)
        726         else:
    --&gt; 727             result = self.forward(*input, **kwargs)
        728         for hook in itertools.chain(
        729                 _global_forward_hooks.values(),

    /opt/anaconda3/lib/python3.7/site-packages/torch/nn/modules/conv.py in forward(self, input)
        421 
        422     def forward(self, input: Tensor) -&gt; Tensor:
    --&gt; 423         return self._conv_forward(input, self.weight)
        424 
        425 class Conv3d(_ConvNd):

    /opt/anaconda3/lib/python3.7/site-packages/torch/nn/modules/conv.py in _conv_forward(self, input, weight)
        418                             _pair(0), self.dilation, self.groups)
        419         return F.conv2d(input, weight, self.bias, self.stride,
    --&gt; 420                         self.padding, self.dilation, self.groups)
        421 
        422     def forward(self, input: Tensor) -&gt; Tensor:

    RuntimeError: Given groups=1, weight of size [16, 1, 3, 3], expected input[16, 3, 1, 28]         to have 1 channels, but got 3 channels instead


I tried a small demo with your code. and it works fine until your code had x = x.view(-1, 64*14*14) and input shape of torch.Size([1, 1, 28 ,28])
import torch
import torch.nn as nn
import torch.nn.functional as F
class Net(nn.Module):
        def __init__(self):
            super(Net, self).__init__()

            self.conv1 = nn.Conv2d(1, 16, 3, padding=1)
            self.conv2 = nn.Conv2d(16, 32, 3, padding=1)
            self.conv3 = nn.Conv2d(32, 64, 3, padding=1)
   
            self.bn1 = nn.BatchNorm2d(16)
            self.pool = nn.MaxPool2d(2, 2)

            unit = 64 * 14 * 14 
            self.fc1 = nn.Linear(unit, 500)
            self.fc2 = nn.Linear(500, 10)
    
        def forward(self, x):
            x = self.pool(F.relu(self.bn1(self.conv1(x))))
            x = F.relu(self.conv2(x))
            x = F.relu(self.conv3(x))
            #print(x.shape)
            x = x.view(-1, 64*14*14)
            x = F.relu(self.fc1(x))
            x = self.fc2(x)
            return x
    

model = Net()
print(model)

data = torch.rand((1,1,28,28))
pred = model(data)

And if i give my data tensor as data = torch.rand((1,3,28,28)) i get your error i.e      RuntimeError: Given groups=1, weight of size [16, 1, 3, 3], expected input[16, 3, 1, 28]         to have 1 channels, but got 3 channels instead
So please check your channel dim of your data just before passing it to your model i.e here  (highlighted by ** **)
for data in train_loader:
        inputs, labels = data[0], data[1]
        optimizer.zero_grad()
        **print(inputs.shape)**
        output = model(inputs)
        loss = criterion(output, labels)
        loss.backward()
        optimizer.step()
        train_loss += loss.item()*data.size(0)

