[Placement Group] Support Placement Group state table. (#10090)

* Done.

* Addressed code review.

* Linting.

* Fix lint.

* Fix lint.

* Fix a test.

* Lint.

* Add a lint sleep to test.

* Fix the lint issue.

* Fixed doc build error.

python/ray/experimental/__init__.py

@@ -1,13 +1,8 @@
 from .api import get, wait
 from .dynamic_resources import set_resource
 from .object_spilling import force_spill_objects, force_restore_spilled_objects
-from .placement_group import (
-    placement_group, )
+from .placement_group import (placement_group, placement_group_table)
 __all__ = [
-    "get",
-    "wait",
-    "set_resource",
-    "force_spill_objects",
-    "force_restore_spilled_objects",
-    "placement_group",
+    "get", "wait", "set_resource", "force_spill_objects",
+    "force_restore_spilled_objects", "placement_group", "placement_group_table"
 ]

python/ray/experimental/placement_group.py

@@ -30,3 +30,10 @@ def placement_group(bundles: List[Dict[str, float]],
         name, bundles, strategy)
 
     return placement_group_id
+
+
+def placement_group_table(placement_group_id):
+    assert placement_group_id is not None
+    worker = ray.worker.global_worker
+    worker.check_connected()
+    return ray.state.state.placement_group_table(placement_group_id)