Make `cog run` and `cog predict` work on GPU images, even if you don't have a GPU

[bfirsh]

This works for cog build but not cog run and cog predict. There's a little thing that checks for the error returned, and re-runs it without --gpus.

Requested by @charlesfrye.

[toddams]

Any update on this? It's been a while :)