Reuse GPUs for multiple ranks in NcclManager test.

Since NCCL 2.1.2, NCCL uses cooperative group launch by default which
results in deadlocks in the NcclManager test if number of ranks > number of
GPUs.  Setting NCCL_LAUNCH_MODE to PARALLEL, which was the default before NCCL
2.1.2, fixes this.

PiperOrigin-RevId: 224531851