[TF:XLA] remove re-initializations of Literals

It's an antipattern to have:

auto x = Literal::CreateFromShape(my_shape);
x->Populate();

as that results in initialization followed by reinitialization. Can be replaced
with:

auto x = MakeUnique<Literal>(my_shape);
x->Populate();

Suggested-by: Kay Zhu <kayzhu@google.com>
PiperOrigin-RevId: 197007127