xla

6e60add5 - Implement distributed executor. (#971)

Implement distributed executor. (#971) Tested on v3-32 and v3-128 (MNIST and ResNet50). (v3-32, ResNet50, fake_data, BF16) logs: https://gist.github.com/jysohn23/34f14f2ab66aedbc33b65b9e200e55d7