def backward(variables, grad_variables, retain_variables=False):
 Variable._execution_engine.run_backward(tuple(variables), tuple(grad_variables), retain_variables)

def backward ( variables , grad_variables , retain_variables = False ) : Variable . _execution_engine . run_backward ( tuple ( variables ) , tuple ( grad_variables ) , retain_variables )
a generator returning lines from a file starting with the last line .