FIX-#337: Fix MpiHosts parameter usage #762

Job	Run time
lint (flake8)	5s
lint (black)	10s
test-ubuntu (backend ray, python 3.8)	38s
test-ubuntu (backend mpi, python 3.8)	39s
test-ubuntu (backend dask, python 3.8)	39s
test-ubuntu (backend pymp, python 3.8)	39s
test-ubuntu (backend pyseq, python 3.8)	38s
test-ubuntu (backend ray, python 3.9)	39s
test-ubuntu (backend mpi, python 3.9)	38s
test-ubuntu (backend dask, python 3.9)	39s
test-ubuntu (backend pymp, python 3.9)	37s
test-ubuntu (backend pyseq, python 3.9)	40s
test-windows (backend ray, python 3.8)	40s
test-windows (backend mpi, python 3.8)	39s
test-windows (backend dask, python 3.8)	2m 18s
test-windows (backend pymp, python 3.8)	40s
test-windows (backend pyseq, python 3.8)	39s
test-windows (backend ray, python 3.9)	1s
test-windows (backend mpi, python 3.9)	38s
test-windows (backend dask, python 3.9)	38s
test-windows (backend pymp, python 3.9)	2m 18s
test-windows (backend pyseq, python 3.9)	38s
	15m 50s

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