Add KokkosComm API to obtain MPI_Datatype from View

[janciesko]

Some library users might like to convert a Kokkos::View to a MPI_Datatype and use that in their existing MPI functions. Therefore it might be beneficial to provide a function that takes a Kokkos::Viewor type T and returns a MPI_Datatype.
Some variations could be:

1: Accepts Kokkos::View

template <typename T>
concept KokkosView = Kokkos::is_view_v<T>;

template <KokkosView View>
MPI_Datatype MPI_Type_create_from_view(const View v);

2: A more generic alternative where T is serializable or convertible and a trait would evaluate to true. Serializable types would provide member functions to create the MPI_Datatype.

template <typename T>
concept SerializableType = KokkosComm::is_serialiazable_v<T>;

template <SerializableType T>
MPI_Datatype MPI_Type_create_from_custom(const T t);

3: User provided type with set of functions to construct the MPI_Datatype.

template <class T>
MPI_Datatype MPI_Type_create_from_custom(const T t, Args... args);

[janciesko]

A reverse path, that is, building Views out of MPI_Datatype should be possible as well, not sure if demand exists.

[devreal]

In general, it would be good if MPI datatypes could persist between KokkosComm operations since creating and committing the type is not free. Then, these functions simply return a std::shared_ptr to that persistent instance of the MPI datatype.

[cedricchevalier19]

A related topic about this kind of memoization is using persistent communications for when a host buffer must be created (for non gpu-aware MPI for example).