SYCL 2020 conformance issue: unconditional use of reqd_sub_group_size attribute

[dmitriy-sobolev]

This is an optional SYCL feature and should be used accordingly. It's a part of the reduce-then-scan algorithm.

There is a warning when using it with acpp - a real use-case when it is not supported:

include/oneapi/dpl/pstl/hetero/dpcpp/parallel_backend_sycl_reduce_then_scan.h:1526:70: warning: unknown attribute 'reqd_sub_group_size' ignored [-Wunknown-attributes]
1526 |                     __nd_range, [=, *this](sycl::nd_item<1> __ndi) [[sycl::reqd_sub_group_size(__sub_group_size)]] {