Fixed issues encountered through oneMKL portBLAS backend

[OuadiElfarouki]

This patch includes following changes :

• Re-enabled half data type support for CPUs the same way as the other devices _(half support no longer disabled when picking tuning_target=defaultcpu)
• Reverted changes (introduced in PR 493) to the BufferIterator<T>::get_range_accessor() that caused some accessors to be not initialized (sycl::property::no_init{}) and thus yield erroneous test results.
• Modify WGAtomicReduction signature to comply with different usm memory allocation and atomic initialization. Adding usmManagedMem to flag if memory is allocated using managed memory which requires generic_space in atomic initialization, especially on AMD hardware. Flag added not to cause performance degradation to the operators that use this kernel.

[pgorlani]

Thanks for your PR, @s-Nick.

Could you please update the PR comment including your modification to WGAtomicReduction?