Implement Detached Actor (#6036)

* Arg propagation works

* Implement persistent actor

* Add doc

* Initialize is_persistent_

* Rename persistent->detached

* Address comment

* Make test passes

* Address comment

* Python2 compatiblity

* Fix naming, py2

* Lint

doc/source/advanced.rst

@@ -235,3 +235,43 @@ To get information about the current available resource capacity of your cluster
 
 .. autofunction:: ray.available_resources
     :noindex:
+
+Detached Actors
+-----------------------------------
+
+When original actor handles goes out of scope or the driver that originally
+created the actor exits, ray will clean up the actor by default. If you want
+to make sure the actor is kept alive, you can use
+``_remote(name="some_name", detached=True)`` to keep the actor alive after
+the driver exits. The actor will have a globally unique name and can be 
+accessed across different drivers. 
+
+For example, you can instantiate and register a persistent actor as follows:
+
+.. code-block:: python
+
+  counter = Counter._remote(name="CounterActor", detached=True)
+
+The CounterActor will be kept alive even after the driver running above script
+exits. Therefore it is possible to run the following script in a different
+driver:
+
+.. code-block:: python
+
+  counter = ray.experimental.get_actor("CounterActor")
+  print(ray.get(counter.get_counter.remote()))
+
+Note that just creating a named actor is allowed, this actor will be cleaned
+up after driver exits:
+
+.. code-block:: python
+
+  Counter._remote(name="CounterActor")
+
+However, creating a detached actor without name is not allowed because there
+will be no way to retrieve the actor handle and the resource is leaked.
+
+.. code-block:: python
+
+  # Can't do this!
+  Counter._remote(detached=True)