[oneMKL][Sparse BLAS] Missing constructor for `oneapi::mkl::sparse::matrix_view`

[gajanan-choudhary]

It appears that we are missing a constructor:

    matrix_view(matrix_descr type_view, uplo uplo_view, diag diag_view);

in the oneapi::mkl::sparse::matrix_view data structure.
While it is possible for users to individually have:

    matrix_view view;
    view.type_view = ...;
    view.uplo_view = ...;
    view.diag_view = ...;

in their code, it would be helpful to allow:

    using namespace oneapi::mkl;
    sparse::matrix_view view{sparse::matrix_descr::triangular, uplo::upper, diag::unit};

Without a triplet constructor, we have errors like so:

foo.cpp:28:87: error: no matching function for call to ‘oneapi::mkl::sparse::matrix_view::matrix_view(<brace-enclosed initializer list>)’
   28 |     sparse::matrix_view view{sparse::matrix_descr::triangular, uplo::upper, diag::unit};
      |                                                                                       ^
foo.cpp:19:9: note: candidate: ‘oneapi::mkl::sparse::matrix_view::matrix_view(oneapi::mkl::sparse::matrix_descr)’
   19 |         matrix_view(matrix_descr type_view);
      |         ^~~~~~~~~~~
foo.cpp:19:9: note:   candidate expects 1 argument, 3 provided
foo.cpp:17:9: note: candidate: ‘constexpr oneapi::mkl::sparse::matrix_view::matrix_view()’
   17 |         matrix_view() = default;
      |         ^~~~~~~~~~~
foo.cpp:17:9: note:   candidate expects 0 arguments, 3 provided
foo.cpp:12:12: note: candidate: ‘constexpr oneapi::mkl::sparse::matrix_view::matrix_view(const oneapi::mkl::sparse::matrix_view&)’
   12 |     struct matrix_view {
      |            ^~~~~~~~~~~
foo.cpp:12:12: note:   candidate expects 1 argument, 3 provided
foo.cpp:12:12: note: candidate: ‘constexpr oneapi::mkl::sparse::matrix_view::matrix_view(oneapi::mkl::sparse::matrix_view&&)’
foo.cpp:12:12: note:   candidate expects 1 argument, 3 provided

@Rbiessy, @spencerpatty, @baeseung-intel

[Rbiessy]

Sure, this constructor can be added easily. We'll get to it when we can.