[FEA]: Return `rmm::device_uvector` directly, not wrapped in std::unique_ptr

[harrism]

Is this a new feature, an improvement, or a change to existing functionality?

Improvement

How would you describe the priority of this feature request

Low (would be nice)

Please provide a clear description of problem you would like to solve.

libcudf's need to return a unique_ptr(cudf::column) due to Cython requirements was blindly copied to returning std::unique_ptr<rmm::device_uvector<T>> in many libcudf functions, and we have blindly copied that habit in the cuSpatial header-only C++ API.

We can easily change these to remove the unique_ptr wrapper. There is no need for unique_ptr in this case since uvectors are not copyable.

Describe any alternatives you have considered

No response

Additional context

No response