print_backprop function print out the module in which the loss will back-propagate
Here is the code
def print_backprop(loss):
    acc = []
    tmp = loss.grad_fn
    while tmp != None:
        try:
            tmp = tmp.next_functions[0][0]
            acc.append(tmp.__str__())
        except:
            break
    print("{}".format(acc))

The result I’ve got is
['<AddBackward1 object at 0x107d03940>', '<AddBackward1 object at 0x107d03470>', 'None']

Also the loss value never change. So I assume that it loss is never propagate back into the network (Also from the print_backprop function).