'Forward interface. Can override when creating new operators.
Parameters
is_train : bool
whether this is for training
req : list of str
how to assign to out_data. can be \'null\', \'write\', or \'add\'.
You can optionally use self.assign(dst, req, src) to handle this.
in_data, out_data, aux: list of NDArrays
input, output, and auxiliary states for forward. See document for
corresponding arguments of Operator::Forward'

def forward(self, is_train, req, in_data, out_data, aux):

    pass
