Could you check your input in the training loop?
Based on the error message, it looks like you are trying to pass a tuple instead of a data tensor.
This might be the case, if you used just one return value for the data and target:
for batch in loader:
    data = batch[0]
    target = batch[1]
    ...

In this example this error would be thrown, if you try to pass batch directly to the model instead of data.