HW acceleration for nntrainer

[myungjoo]

We may need to start considering hardware acceleration for nntrainer by validating designs for different accelerators including GPUs and NPUs.

For GPU, openCL might be considered. However, let's see if there are other options before we start actual work. I.e., https://github.com/KomputeProject/kompute

Let's start with random ideas, requirements, other packages to examine, ...