[ray] Default gpu_resources_per_trial to 1 when GPUs are available

[tgaddair]

Currently, we default gpu_resources_per_trial to 0 if unspecified (see here). However, this is counter-intuitive for users running on a GPU clusters. They would most of the time prefer to have it default to something like 1 GPU per trial.

We've already accounted for this somewhat in automl (here) by setting the config to use GPU when available, but we should take this one step further and just make it the default when not specified in the config.

If the user specifies 0 explicitly, it should be honored by not requesting GPUs.