'Backward interface. Can override when creating new operators.
Parameters
req : list of str
how to assign to in_grad. can be \'null\', \'write\', or \'add\'.
You can optionally use self.assign(dst, req, src) to handle this.
out_grad, in_data, out_data, in_grad, aux : list of NDArrays
input and output for backward. See document for
corresponding arguments of Operator::Backward'

def backward(self, req, out_grad, in_data, out_data, in_grad, aux):

    pass
