Fix nonzero for scalars on cuda, to_sparse for scalars on cpu/cuda. (#17406)

Fix nonzero for scalars on cuda, to_sparse for scalars on cpu/cuda. (#17406)

Summary:
I originally set out to fix to_sparse for scalars, which had some overly restrictive checking (sparse_dim > 0, which is impossible for a scalar).

This fix uncovered an issue with nonzero: it didn't properly return a size (z, 0) tensor for an input scalar, where z is the number of nonzero elements (i.e. 0 or 1).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17406

Differential Revision: D14185393

Pulled By: gchanan

fbshipit-source-id: f37a6e1e3773fd9cbf69eeca7fdebb3caa192a19