Remove the usages of __future on internal layers

[SergeyKopienko]

In this PR we remove the usage of __future class on internal layers of oneDPL code.

Instead of them now we using:

• std::tuple<sycl::event>;
• std::tuple<sycl::event, __result_and_scratch_storage>;
• std::tuple<sycl::event, __result_and_scratch_storage_base_ptr>.
  These types are enough for implementation of required functional.

Now we create __future instances only in experimental async algorithm implementations.

Also in this PR we introduced new type __result_and_scratch_storage_base_ptr and declare it as

using __result_and_scratch_storage_base_ptr = std::unique_ptr<__result_and_scratch_storage_base>;

So now we are using __result_and_scratch_storage_base_ptr type instead of std::shared_ptr<__result_and_scratch_storage_base> in all code.

[SergeyKopienko]

@danhoeflinger, I think you may continue thinks about re-design of __future class and so on.
On internal layers this class doesn't needed anymore.

[MikeDvorskiy]

First, I have a couple of questions:

1. Which issue does PR solve?
2. This is a significant architectural change. Was there a design review for a wide audience?

[SergeyKopienko]

First, I have a couple of questions:

1. Which issue does PR solve?
2. This is a significant architectural change. Was there a design review for a wider audience?

1. Which issue does PR solve?
   __future is something that make sense only for async experimental implementations. On internal layers it's not required, I believe.

2. This is a significant architectural change. Was there a design review for a wider audience?
   Yes, it's planned.

[Copilot]

Pull Request Overview

This PR removes the internal usage of the __future type by replacing it with tuple-based returns or direct event returns throughout the oneDPL hetero backend. The changes update various algorithm implementations and backend utilities so that asynchronous kernel submissions now return a std::tuple or sycl::event instead of a __future.

• Removed __future wrapping and replaced with std::tuple<sycl::event, storage> patterns.
• Updated return types and related documentation/comments in several hetero backend files.
• Switched shared_ptr types to the new __result_and_scratch_storage_base_ptr for memory lifetime management.

Reviewed Changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated no comments.

File	Description
include/oneapi/dpl/pstl/hetero/numeric_ranges_impl_hetero.h	Updated __parallel_transform_reduce call to return a tuple and use __wait_and_get_value.
include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_utils.h	Removed internal __future methods and updated wait implementations.
include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_merge_sort.h	Changed shared_ptr usage to __result_and_scratch_storage_base_ptr.
...	Other related files updated to use tuple returns or direct sycl::event.

Comments suppressed due to low confidence (3)

include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_utils.h:736

• Update the inline documentation for __wait_and_get_value to reflect that it is now used in tandem with the new __result_and_scratch_storage_base_ptr based return pattern instead of __future.

std::pair<std::size_t, std::size_t> __wait_and_get_value(const std::shared_ptr<__result_and_scratch_storage_base>& __p_storage)

include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_merge_sort.h:591

• Ensure that the naming convention for the new type __result_and_scratch_storage_base_ptr is applied consistently throughout the file after replacing shared_ptr, to avoid confusion in the API.

std::shared_ptr<__result_and_scratch_storage_base> __p_result_and_scratch_storage_base;

include/oneapi/dpl/pstl/hetero/numeric_ranges_impl_hetero.h:60

• Confirm that using __storage.__wait_and_get_value(__event) in place of the previous get() call maintains the expected asynchronous behavior without introducing unintended blocking, and that it integrates seamlessly with the tuple-based return conventions.

return __storage.__wait_and_get_value(__event);