[oneMKL][DFT] Copying and moving the dft::descriptor class

[hjabird]

The oneMKL descriptor class has its constructor / destructor / copy / move interface defined as follows:

namespace oneapi::mkl::dft {
   template <oneapi::mkl::dft::precision prec, oneapi::mkl::dft::domain dom>
   class descriptor {
    public:
       descriptor(std::int64_t length);
       descriptor(std::vector<std::int64_t> dimensions);
       ~descriptor();
    };
}

Consequently one might expect:

• Copy constructor and copy assignment - should be present.
• Move constructor and assignment - I think these are implicitly deleted due to the user declared destructor.

In the current implementation of oneMKL, the implicit copy assignment / constructors are deleted due to the unique_ptr to the backend-specific descriptor implementation class.

For copying, it isn't clear what the behavour should be:

• If constructor was copied, except for the backend-specific implementation class, and then recommitted, copying would become a very expensive operation.
• Throwing on copying an already committed constructor at runtime would be annoying.

I don't see any downsides to move constructors / assignment operators.

What should the behaviour of the copy constructor be, and should there be move constructors?

[hjabird]

#490 resolves this issue.