[Placement Group] Capture child tasks by default. (#11025)

* In progress.

* Finished up.

* Improve comment.

* Addressed code review.

* Fix test failure.

* Fix ci failures.

* Fix CI issues.

python/ray/runtime_context.py

@@ -54,6 +54,18 @@ class RuntimeContext(object):
         """
         return self.worker.placement_group_id
 
+    @property
+    def should_capture_child_tasks_in_placement_group(self):
+        """Get if the current task should capture parent's placement group.
+
+        This returns True if it is called inside a driver.
+
+        Returns:
+            Return True if the current task should implicitly
+                capture the parent placement group.
+        """
+        return self.worker.should_capture_child_tasks_in_placement_group
+
 
 _runtime_context = None