[Graphbolt] [NFC] Remove default backend torch.distributed.init_process_group calls

With newer pytorch versions, the behavior in these locations was changed from initializing nccl+gloo to only initializing gloo. Now, all locations explicitly specify their desired backends. These additions maintain expected torch<=2.4 behavior on torch>=2.6. The changed test relies on both backends existing and would fail on newer torch versions. See https://github.com/pytorch/pytorch/issues/147631