[04-Ray-Multiprocessing] 'BatchedCalls' object has no attribute '_reducer_callback'

[junteken]

Running the function ('with joblib.parallel_backend('ray'):') causes following error.

error message :
/usr/local/lib/python3.6/dist-packages/ray/util/multiprocessing/pool.py in _convert_to_ray_batched_calls_if_needed(self, func)
621 if isinstance(func, BatchedCalls):
622 func = RayBatchedCalls(func.items, (func._backend, func._n_jobs),
--> 623 func._reducer_callback, func._pickle_cache)
624 # go through all the items and replace args and kwargs with
625 # ObjectRefs, caching them in registries

AttributeError: 'BatchedCalls' object has no attribute '_reducer_callback'