[oneDPL] Declare predefined dpcpp_default policy as const

[SergeyKopienko]

(Edited by @akukanov)
This patch fixes what we consider to be a bug in the oneDPL specification, namely - the predefined dpcpp_default execution policy not being const.

Since all explicitly specified member functions of the device_policy class are const, a mutable policy object differs from a non-mutable (const) policy object in two ways:

• when an lvalue, it can be a target to assign (i.e. copy or move) another policy object to, changing the associated SYCL queue to the queue associated with the other policy;
• when an rvalue, it can be moved to another policy object, which may invalidate the associated queue and leave the moved-from policy in an unspecified state.

Neither of the above is an appropriate modification for dpcpp_default, potentially changing its semantics as a predefined policy associated with the default SYCL device. The possibility of such modifications was previously overlooked and deserves to be fixed.

The change will not impact the ability to pass dpcpp_default to oneDPL algorithms, to obtain its associated queue, and to copy or rebind the policy (including using it as the argument to the make_device_policy function), which constitute the vast majority of its usages in application codes. Codes that might be broken by the change either do not properly handle any const instances of the class or depend on the possibility to modify dpcpp_default, which we do not consider as a proper use and want to prevent. In the latter case, a workaround can be to create a copy of dpcpp_default and use that instead.

[SergeyKopienko]

@akukanov Why fpga_policy is not described in our specification?

[akukanov]

Why fpga_policy is not described in our specification?

There is neither enough implementation experience nor enough demand to justify adding special FPGA policies to the oneDPL specification.

[danhoeflinger]

Lets mark the title of this PR with [oneDPL] to match the convention established so far.

[SergeyKopienko]

Lets mark the title of this PR with [oneDPL] to match the convention established so far.

Done.

[danhoeflinger]

LGTM