Multi-GPU system - specify which to use? query for which is being used?


## *What* is wrong?

I have a muli-gpu rig setup and I'd really like to make sure my gpujs gpu instance is taking advantage of the most powerful GPU in my system for ML Training but I would also like to experiment with running the model on a lower powered gpu also available on my rig.

## *Where* does it happen?

I dont see anything in the api that would support being able to select a specific gpu or even get a list.

## *How* do we replicate the issue?

## *How* important is this (1-5)?

3.5 (not a deal breaker but would be a strong nice to have)

## Expected behavior (i.e. solution)

If there was api support to fetch a list of available gpus and to be able to specify when creating a Gpu instance which physical gpu to use that would meet my needs.

## Other Comments
I really like GPU.js - great work all around!