[SYCL][NFC] Doxygen doc for CUDA backend (#1394)

Adding doxygen documentation to PI CUDA backend.
Some code is re-ordered in the file to help sorting the
doxygen.

Co-Authored-By: Alexey Bader <[email protected]>
Co-Authored-By: Alexander Batashev <[email protected]>
Co-Authored-By: Romanov Vlad <[email protected]>

Signed-off-by: Ruyman Reyes <[email protected]>

Author: Ruyk