[Scalars] Remove Scalar from return types of functions.

[zdevito]

We want to unite functions like Scalar sum(Tensor & tensor); with the other variants of sum. To do this, we must return a Tensor in these cases.

This can be done in two parts:
[ ] Force the creation of Tensors from Scalar objects now.
[ ] Change TH so that in cases where scalars exist on the GPU, we don't both to copy back and just create a Tensor directly.

Note: the first part depends on changing the behavior of how 0-dim tensors in all places where they are valid.

[killeent]

Per @gchanan this is fixed.