'Backward propagate lookup table layer.
Arguments:
nin (int): Number of input word_ids.
inputs (Tensor): Input tensor.
error (Tensor): Error tensor.
error_t (Tensor): Transposed error tensor.
dW (Tensor): Gradient tensor (delta).
pad_idx (int):
alpha (float):
beta (float):'

@abc.abstractmethod
def compound_bprop_lut(self, nin, inputs, error, error_t, dW, pad_idx, alpha=1.0, beta=0):

    raise NotImplementedError()
