xla

01aa6b66 - Add client side compitation cache.

Add client side compitation cache. This reduces compile time in many ways. The ClientSession::Run() method needs to serialize the request to proto, which for big computation size objects can be a few ms. The GRPC latency can vary in the range of few ms as well. On the service side, the compilation cache is distributed, which requires network lookup. The de-serialization on the service side for big computation objects can be in the few ms as well. For reference, each of the 8 parallel computations we issue for resnet50 is about 500KB in size.