*   Visualization of embeddings in TensorBoard.

## Breaking Changes to the API

*   `BusAdjacency` enum replaced with a protocol buffer `DeviceLocality`. PCI
    bus indexing now starts from 1 instead of 0, and `bus_id==0` is used where
    previously `BUS_ANY` was used.
*   `Env::FileExists` and `FileSystem::FileExists` now return a
    tensorflow::Status instead of a bool. Any callers to this function can be