[SYCL] Restore the nd_range default constructor

[arnavprabhu]

Fixes #22940

Problem

PR #22908 removed nd_range's default constructor (nd_range() = default;)
to align the class with the SYCL 2020 specification, which does not declare
one. That broke code which default-constructs an nd_range and assigns it
later. The most visible casualty is CuTe's launch_policy
(include/cute/util/compat/launch_policy.hpp), used by PyTorch's XPU
flash-attention kernels, which holds an nd_range<3> member that gets
default-initialized:

error: constructor for 'compat::experimental::launch_policy<sycl::nd_range<3>, ...>'
must explicitly initialize the member '_range' which does not have a default constructor

Change

Restore nd_range() = default;. All three members (globalSize, localSize,
offset) are range/id objects, both of which keep zero-initializing
default constructors, so a default-constructed nd_range is well-defined and
produces zeroed ranges and offset.

Test plan

• Extended sycl/test/basic_tests/nd_range.cpp to default-construct an
  nd_range<3> and assert the constituent ranges and offset are
  zero-initialized.

[KornevNikita]

Note for reviewers - please do not merge it right now. We need a few days to discuss this and decide if we want to put it to SYCL 2020 (most likely) or to create a oneapi extension.

[dvrogozh]

if we want to put it to SYCL 2020 (most likely) or to create a oneapi extension.

Please, keep me updated on this.