mpi: fix allreduce iteration space #8214

Job	Run time
pytest-ubuntu-py311-gcc11-cxxnoomp-base	44m 28s
pytest-ubuntu-py311-gcc11-cxxnoomp-adjoint	32m 15s
pytest-ubuntu-py312-gcc12-cxxomp-base	56m 1s
pytest-ubuntu-py311-gcc10-noomp-base	40m 36s
pytest-ubuntu-py312-gcc12-cxxomp-adjoint	38m 23s
pytest-ubuntu-py310-gcc14-omp-adjoint	32m 11s
pytest-ubuntu-py310-gcc14-omp-base	44m 25s
pytest-ubuntu-py310-gcc9-omp-base	44m 18s
pytest-ubuntu-py312-gcc13-omp-base	53m 52s
pytest-docker-py310-gcc-omp-base	37m 45s
pytest-ubuntu-py310-gcc9-omp-adjoint	32m 36s
pytest-osx-py312-clang-omp-base	29m 1s
pytest-docker-py310-icx-omp-base	32m 12s
pytest-ubuntu-py311-gcc10-noomp-adjoint	30m 18s
pytest-docker-py310-gcc-omp-adjoint	29m 2s
pytest-docker-py310-icx-omp-adjoint	29m 14s
pytest-ubuntu-py312-gcc13-omp-adjoint	36m 57s
	10h 43m 34s

Provide feedback