I’m sorry for the confusion. The second column is the label column which consists of zeros and ones. In the debugged part (until the error), all labels are 0 by a coincidence, therefore I thought there was a mistake. I don’t know why but I am not able to edit my first message.
Concisely, this tensor seems okay but the problem is still the error below:

RuntimeError: input.size(-1) must be equal to input_size. Expected 1, got 2

My training data contains only one feature column, therefore I pass “n_features” = 1. Labels can only be 0 or 1, hence this is binary classification, I pass “n_classes” = 1. With this setup, I get the error above.
If and only if I pass “n_features” = 2 and “n_classes” = 2, the model works.
I would be grateful if you help me on this. Thanks @ptrblck !