Groupby tests on MPI are failing

[dchigarev]

If you would run:

MODIN_ENGINE=unidist MODIN_CPUS=2 mpiexec -n 1 python -m pytest modin/pandas/test/test_groupby.py::test_groupby_sort[True-True] -v

You would get this error:

modin/pandas/test/test_groupby.py::test_groupby_sort[True-True] Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "miniconda3/envs/modinn/lib/python3.9/site-packages/unidist/api.py", line 92, in init
    init_backend()
  File "miniconda3/envs/modinn/lib/python3.9/site-packages/unidist/core/base/utils.py", line 28, in init_backend
    initialize_mpi()
  File "miniconda3/envs/modinn/lib/python3.9/site-packages/unidist/core/backends/mpi/utils.py", line 12, in initialize_mpi
    init()
  File "miniconda3/envs/modinn/lib/python3.9/site-packages/unidist/core/backends/mpi/core/controller/api.py", line 278, in init
    asyncio.run(worker_loop())
  File "miniconda3/envs/modinn/lib/python3.9/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "miniconda3/envs/modinn/lib/python3.9/asyncio/base_events.py", line 647, in run_until_complete
    return future.result()
  File "miniconda3/envs/modinn/lib/python3.9/site-packages/unidist/core/backends/mpi/core/worker/loop.py", line 111, in worker_loop
    pending_request = task_store.process_task_request(request)
  File "miniconda3/envs/modinn/lib/python3.9/site-packages/unidist/core/backends/mpi/core/worker/task_store.py", line 423, in process_task_request
    task = request["task"]
KeyError: 'task'