enable Half in mpi

[yhmtsai]

This PR enables half precision in distributed environment by adding custom operation.

one-side operation like accumulation and fetch_and_op does not support custom operation.

Note. Newer version of mpi might support half precision natively (also for one-side operation) if the administrator build it with compiler supporting native half precision and enable the option.

TODO:

• enable the rest distributed function with half
• put the custom operation in gko::comm?

[ginkgo-bot]

Error: The following files need to be formatted:

common/cuda_hip/distributed/assembly_kernels.cpp
common/cuda_hip/distributed/matrix_kernels.cpp
common/cuda_hip/distributed/vector_kernels.cpp
common/unified/distributed/assembly_kernels.cpp
core/device_hooks/common_kernels.inc.cpp
core/distributed/assembly.cpp
core/distributed/helpers.hpp
core/distributed/matrix.cpp
core/distributed/preconditioner/schwarz.cpp
core/distributed/vector.cpp
core/test/mpi/base/bindings.cpp
core/test/utils.hpp
dpcpp/distributed/assembly_kernels.dp.cpp
dpcpp/distributed/matrix_kernels.dp.cpp
dpcpp/distributed/vector_kernels.dp.cpp
include/ginkgo/core/base/mpi.hpp
include/ginkgo/core/distributed/matrix.hpp
include/ginkgo/core/distributed/vector.hpp
omp/distributed/assembly_kernels.cpp
omp/distributed/matrix_kernels.cpp
omp/distributed/vector_kernels.cpp
reference/distributed/assembly_kernels.cpp
reference/distributed/matrix_kernels.cpp
reference/distributed/vector_kernels.cpp
test/mpi/solver/solver.cpp
test/mpi/vector.cpp

You can find a formatting patch under Artifacts here or run format! if you have write access to Ginkgo