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
    converted to a bool by adding .ok() to the call.
*   The C API type `TF_SessionWithGraph` has been renamed to `TF_Session`,