[Feature Request] Report used and available GPUs with partition_info()

[Nick-Eagles]

Consider adding "Gres,GresUsed:50" to the sinfo -O piece here. This is intended to report available and used GPUs for GPU nodes at JHPCE. Ultimately parse that into something more accessible (e.g. number of free GPUs). Since this is somewhat likely to be specific to GPU partitions at JHPCE, potentially just don't add this extra info on non-JHPCE, non-GPU partitions.