[SYCL][COMPAT] Add dummy template params to memory APIs to delay kernel instantiation (#17743)

The functions `syclcompat::detail::fill` and
`syclcompat::detail::memcpy` define SYCL kernels. Any translation unit
including the `memory.hpp` or `util.hpp` headers will contain these
kernels, as DPC++'s 2-pass compiler is not currently able to reason
about which kernels are actually used.

Adding a dummy template parameter (typename T = void) ensures that these
functions (and thus their kernels) are only instantiated if used.

Author: joeatodd