Let actors use GPUs. (#302)

* Add num_cpus and num_gpus to actor decorator.

* Assign GPU IDs to actors.

* Add additional actor test.

* Remove duplicated line.

* Factor out local scheduler selection method.

* Add test and simplify local scheduler selection.

python/ray/__init__.py

@@ -17,5 +17,6 @@ import ray.experimental
 import ray.serialization
 from ray.worker import register_class, error_info, init, connect, disconnect, get, put, wait, remote, log_event, log_span, flush_log
 from ray.actor import actor
+from ray.actor import get_gpu_ids
 from ray.worker import EnvironmentVariable, env
 from ray.worker import SCRIPT_MODE, WORKER_MODE, PYTHON_MODE, SILENT_MODE