Why are these changes needed?

The check failure can happen under the following sequence of events:

1. An async or threaded actor is launched.
2. An actor task is submitted.
3. Actor task is received by the actor and is being executed. The task_id is added to CoreWorker.current_tasks_.
4. A transient network error happens and caller retries the actor task.
5. The second attempt of the actor task is received by the actor and is being executed.
6. The first attempt of the actor task finishes and the task_id is removed from CoreWorker.current_tasks_.
7. The second attempt of the actor task finishes and the check failure happens when we try to erase the same task_id from CoreWorker.current_tasks_ again:

auto it = current_tasks_.find(task_spec.TaskId());
RAY_CHECK(it != current_tasks_.end());
current_tasks_.erase(it);

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(