mpi: fix allreduce iteration space #8216

Job	Run time
pytest-ubuntu-py311-gcc11-cxxnoomp-adjoint	7m 30s
pytest-ubuntu-py311-gcc11-cxxnoomp-base	1m 34s
pytest-ubuntu-py312-gcc12-cxxomp-base	2m 4s
pytest-ubuntu-py312-gcc13-omp-base	2m 2s
pytest-ubuntu-py311-gcc10-noomp-base	1m 39s
pytest-ubuntu-py310-gcc9-omp-adjoint	7m 31s
pytest-ubuntu-py311-gcc10-noomp-adjoint	6m 12s
pytest-osx-py312-clang-omp-base	2m 35s
pytest-ubuntu-py310-gcc14-omp-base	1m 53s
pytest-docker-py310-gcc-omp-base	2m 8s
pytest-ubuntu-py310-gcc14-omp-adjoint	7m 22s
pytest-ubuntu-py310-gcc9-omp-base	1m 50s
pytest-docker-py310-icx-omp-adjoint	7m 21s
pytest-ubuntu-py312-gcc12-cxxomp-adjoint	7m 49s
pytest-ubuntu-py312-gcc13-omp-adjoint	7m 23s
pytest-docker-py310-icx-omp-base	3m 9s
pytest-docker-py310-gcc-omp-adjoint	7m 4s
	1h 17m 6s

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