If aux_vec is not provided, assume that we shouldn't compute aux attention.
If aux_vec is not provided, assume that we shouldn't compute aux attention.

Currently it is assumed that if has_aux_atten is True, that the user will
always provide aux_vec.  But there are cases where at runtime, the caller
may not provide an aux_vec for evaluation, even though the model has
set has_aux_vec to True.

PiperOrigin-RevId: 421889180

tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo,tensorflow/lingvo