mixpanel: improved GPU device statistics (plus GPU sort order fix)

[cebtenzzre]

Added properties to model_load event:

• default_device: what GPU would have been chosen if "Auto" were selected (unset if none available)
• default_device_mem: how much VRAM said GPU has in GiB, rounded down to the nearest 100MiB (e.g. 3.2)
• requested_device_mem: how much VRAM the device the user has requested has in GiB

Other changes:

• Pick the GPU with the most VRAM by default, not the least (GPT4All has been wanting to use my GTX 970 instead of my P40 until now)
• Do not search for a device named "Metal" on macOS devices (there won't be any devices anyway)
• If somehow the requested GPU does not exist, use the default GPU instead of the CPU (shouldn't happen in practice but it makes the code simpler)