[core] actor object not garbage collected when being killed

[raulchen]

Sometimes an actor can depend on external resources that need be cleaned up upon actor termination. and users would want to do the clean up in Actor.__del__.

But currently the actor object is referenced by global_worker.actors, thus Actor.__del__ won't be called.

We should add a callback in CoreWorker::HandleKillActor to remove the reference.

Repro:

import time
import ray


@ray.remote
class Actor:
    def __init__(self):
        print("Actor.__init__")

    def ping(self):
        # Without the following, `__del__` won't be called.
        worker = ray._private.worker.global_worker
        actor_id = worker.actor_id
        del worker.actors[actor_id]
        return ""

    def __del__(self):
        print("Actor.__del__")


actor = Actor.remote()
ray.get(actor.ping.remote())

del actor
time.sleep(3)

Related issue for Ray Data: #49929