[core][experimental] Support tasks with multiple return values in accelerated DAG

[stephanie-wang]

Description

Accelerated DAGs currently error if a task with multiple return values is found.

In [11]: @ray.remote
    ...: class Actor:
    ...:     @ray.method(num_returns=2)
    ...:     def forward(self, x):
    ...:         return x, x + 1
    ...: 

In [12]: a = Actor.remote()

In [13]: with ray.dag.InputNode() as inp:
    ...:     dag = a.forward.bind(inp)
    ...: 

In [14]: dag.execute(0)
Out[14]: 
[ObjectRef(32d950ec0ccf9d2ad9e7e77e45dee424649f688f0100000001000000),
 ObjectRef(32d950ec0ccf9d2ad9e7e77e45dee424649f688f0100000002000000)]

In [15]: ray.get(dag.execute(0))
Out[15]: [0, 1]

In [16]: dag = dag.experimental_compile()

In [17]: ray.get(dag.execute(0))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[17], line 1
----> 1 ray.get(dag.execute(0))

File ~/ray/python/ray/_private/auto_init_hook.py:21, in wrap_auto_init.<locals>.auto_init_wrapper(*args, **kwargs)
     18 @wraps(fn)
     19 def auto_init_wrapper(*args, **kwargs):
     20     auto_init_ray()
---> 21     return fn(*args, **kwargs)

File ~/ray/python/ray/_private/client_mode_hook.py:103, in client_mode_hook.<locals>.wrapper(*args, **kwargs)
    101     if func.__name__ != "init" or is_client_mode_enabled_by_default:
    102         return getattr(ray, func.__name__)(*args, **kwargs)
--> 103 return func(*args, **kwargs)

File ~/ray/python/ray/_private/worker.py:2617, in get(object_refs, timeout)
   2614     object_refs = [object_refs]
   2616 if not isinstance(object_refs, list):
-> 2617     raise ValueError(
   2618         f"Invalid type of object refs, {type(object_refs)}, is given. "
   2619         "'object_refs' must either be an ObjectRef or a list of ObjectRefs. "
   2620     )
   2622 # TODO(ujvl): Consider how to allow user to retrieve the ready objects.
   2623 values, debugger_breakpoint = worker.get_objects(object_refs, timeout=timeout)

ValueError: Invalid type of object refs, <class 'ray.experimental.channel.SynchronousReader'>, is given. 'object_refs' must either be an ObjectRef or a list of ObjectRefs. 

Use case

No response

[rkooo567]

we can work around it with tuple for now I think. Let's revisit it after finishing existing P1s?