Add MPIGradient #265

Job	Run time
build (ubuntu-latest, 3.8, openmpi)	1m 59s
build (ubuntu-latest, 3.8, intelmpi)	2m 9s
build (ubuntu-latest, 3.9, openmpi)	2m 6s
build (ubuntu-latest, 3.9, intelmpi)	2m 2s
build (ubuntu-latest, 3.10, mpich)	2m 6s
build (ubuntu-latest, 3.10, openmpi)	2m 7s
build (ubuntu-latest, 3.10, intelmpi)	1m 59s
build (ubuntu-latest, 3.11, mpich)	2m 24s
build (ubuntu-latest, 3.11, openmpi)	2m 19s
build (ubuntu-latest, 3.11, intelmpi)	2m 4s
build (macos-latest, 3.8, mpich)	4m 23s
build (macos-latest, 3.8, openmpi)	2m 49s
build (macos-latest, 3.9, mpich)	2m 37s
build (macos-latest, 3.9, openmpi)	6m 39s
build (macos-latest, 3.10, mpich)	3m 5s
build (macos-latest, 3.10, openmpi)	2m 47s
build (macos-latest, 3.11, mpich)	2m 51s
build (macos-latest, 3.11, openmpi)	3m 6s
	49m 32s

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