[PjRt] Implement `xm.rendezvous` for TPU v4. (#3929)
* Implement `xm.rendezvous` for TPU v4.
* Set default for torch.distributed in PJRT test.
* Add mesh_reduce back to model tests.
* Initialize process group with file if world size is 1.
* Take mesh reduce back out of model tests because it will break v3.
