mpi: fix allreduce iteration space #8237

Job	Run time
pytest-ubuntu-py311-gcc10-noomp-base	32m 26s
pytest-ubuntu-py311-gcc11-cxxnoomp-base	35m 20s
pytest-ubuntu-py312-gcc12-cxxomp-base	44m 29s
pytest-ubuntu-py310-gcc9-omp-base	35m 8s
pytest-ubuntu-py310-gcc14-omp-base	34m 51s
pytest-osx-py312-clang-omp-base	23m 41s
pytest-ubuntu-py310-gcc14-omp-adjoint	33m 14s
pytest-ubuntu-py311-gcc11-cxxnoomp-adjoint	32m 33s
pytest-ubuntu-py312-gcc12-cxxomp-adjoint	39m 37s
pytest-ubuntu-py310-gcc9-omp-adjoint	32m 52s
pytest-ubuntu-py311-gcc10-noomp-adjoint	30m 4s
pytest-ubuntu-py312-gcc13-omp-adjoint	36m 41s
pytest-ubuntu-py312-gcc13-omp-base	41m 14s
pytest-ubuntu-py313-gcc14-omp-adjoint	40m 1s
pytest-docker-py310-gcc-omp-base	29m 45s
pytest-docker-py310-gcc-omp-adjoint	27m 39s
pytest-ubuntu-py313-gcc14-omp-base	42m 31s
pytest-docker-py310-icx-omp-adjoint	28m 16s
pytest-docker-py310-icx-omp-base	23m 26s
	10h 43m 48s

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