add default_cpu_device() and default_gpu_device() to complement find def...

add default_cpu_device() and default_gpu_device() to complement find default_device()

I found myself often wanting to quickly compare things on my cpu vs. my gpu, the best way to do this (that I could think of) was to know ahead of time what your device name was and use find device.

I think this is a good solution for this but would love some feedback.

thanks

Dan