[Sparse] Move `SparseTensorUtils.*` to `native/` (#96696)
Fixes internal linking problem after `DECLARE_DISPATCH` was introduced in SparseTensorUtils.cpp, but implemented inside the native library.
Also, fix `sign-unsigned` compare in `_flatten_indices_impl`
Followups:
Move code declared/implemented in `SparseTensorUtils.*` to `at::native` namespace
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96696
Approved by: https://github.com/albanD
