Solved by user_123454321 in post #3 
              
              
                Usually people use 2 ways to get around this 

Take the tensor of shape (C=1, H, W) into (C=3, H, W) by concating the tensor in the channel dimensions 3 times.
Use a conv2d before passing to the model with input channels 1 and output channels 3 with kernel size 1 so that one gets a 3 channel input w…