Have EnsureBiasVector create bias vectors that already have the

constant value 0 and already have their shape set from the output
activations shape; instead of having it create dummy placeholders
and relying on PropagateFixedSizes to create the constant array.
Rationale: It wasn't PropagateFixedSizes's job to create constant
arrays, and that broke down in a case where the bias vectors not
being constant prevented FuseBinaryIntoPrecedingAffine from running.

PiperOrigin-RevId: 202120850