Revert "[rllib] remove exists call (#6168)" (#6616)

This reverts commit a68cda0a33.

rllib/utils/actors.py

@@ -40,12 +40,16 @@ class TaskPool(object):
         Assumes obj_id only is one id."""
 
         for worker, obj_id in self.completed(blocking_wait=blocking_wait):
+            (ray.worker.global_worker.raylet_client.fetch_or_reconstruct(
+                [obj_id], True))
             self._fetching.append((worker, obj_id))
 
         remaining = []
         num_yielded = 0
         for worker, obj_id in self._fetching:
-            if num_yielded < max_yield:
+            if (num_yielded < max_yield
+                    and ray.worker.global_worker.core_worker.object_exists(
+                        obj_id)):
                 yield (worker, obj_id)
                 num_yielded += 1
             else: