mpi: fix allreduce iteration space #8217

Job	Run time
pytest-ubuntu-py312-gcc12-cxxomp-base	56m 0s
pytest-ubuntu-py311-gcc11-cxxnoomp-adjoint	32m 7s
pytest-ubuntu-py311-gcc11-cxxnoomp-base	44m 33s
pytest-ubuntu-py312-gcc12-cxxomp-adjoint	37m 24s
pytest-ubuntu-py310-gcc14-omp-base	44m 31s
pytest-ubuntu-py310-gcc14-omp-adjoint	32m 35s
pytest-ubuntu-py310-gcc9-omp-adjoint	32m 47s
pytest-ubuntu-py310-gcc9-omp-base	43m 56s
pytest-docker-py310-gcc-omp-base	37m 33s
pytest-ubuntu-py311-gcc10-noomp-base	40m 42s
pytest-osx-py312-clang-omp-base	27m 56s
pytest-ubuntu-py312-gcc13-omp-base	51m 43s
pytest-ubuntu-py311-gcc10-noomp-adjoint	29m 33s
pytest-docker-py310-icx-omp-base	30m 49s
pytest-ubuntu-py312-gcc13-omp-adjoint	37m 13s
pytest-docker-py310-icx-omp-adjoint	27m 52s
pytest-docker-py310-gcc-omp-adjoint	28m 4s
	10h 35m 18s

Provide feedback