[SPMD] Support reduce-scatter in manual sharding (#7231)
Summary:
This PR is to add experimental support of cc ops in manual sharding zones. This one adds reduce-scatter as the initial step. The key here is to add channel_id, replica_groups, and use_global_device_ids in the lowering.
Test Plan:
PJRT_DEVICE=TPU XLA_USE_SPMD=1 python test/spmd/test_xla_sharding.py -v -k test_spmd_reduce_scatter
