xla

021a1cc2 - [PJRT] Experimental support for `torch.distributed` and DDP on TPU v2/v3 (#4520)

[PJRT] Experimental support for `torch.distributed` and DDP on TPU v2/v3 (#4520) * Implement multithreaded XLA process group * Fix tests * Merge PJRT MNIST test * formatting * Clarify random generation in test_ddp.py * Mark some variables private * Remove some extra comments * Add test that uses env:// method * Explain local RNG * Explain --pjrt_distributed flag