MNIST error

I’m studying DL subject at my college.
I want to use fastai to complete my exercise, but I get an error.
Please help me to fix it.

Try switching to the CPU rather than the GPU to see if the problem disappears. You can also try Googling the message and see whether the dimension mis-match fits your problem.
Regards Conwyn

I use CELoss and I don’t get errors. Why my teacher use BCELoss for MNIST?