[core][experimental] Propagate leaf node errors to users

[kevin85421]

Description

See this gist for more details. If a DAG node is not an ancestor of the MultiOutputNode, errors thrown by the DAG node will not be propagated to users.

In the following example, the assertion error of a.fail will not be propagated to users.

@ray.remote
class Actor:
  ...
  def fail(self, x):
    assert x != x 

def test_leaf_node_one_actor(self, ray_start_regular):
    a = Actor.remote(0)
    with InputNode() as i:
        input_data = a.read_input.bind(i)
        branch1 = a.fail.bind(input_data)  # branch1: leaf node
        branch2 = a.inc.bind(input_data)
        dag = MultiOutputNode([branch2])

    compiled_dag = dag.experimental_compile()

    ref = compiled_dag.execute(10)
    print(ray.get(ref))
    compiled_dag.teardown()

Possible solution is to add leaf node to MultiOutputNode implicitly.

Use case

No response