message: Remove host_spec attr from TPU configuration ops since it isn't used any more.
diff: @@ -45,10 +45,8 @@ using shape_inference::ShapeHandle;
  //
  // 4 Run _WaitForDistributedTPU on TPU_SYSTEM, taking as input the
  // outputs from all the _InitializeHostForDistributedTPU
  -// Ops. _WaitForDistributedTPU has an attr host_specs which is a
  -// vector<string> giving the partial device spec for each host. These
  -// partial specs are combined in the Op with the outputs from the host
  -// initialization Ops to construct a mapping from full TPU device
  +// Ops. _These partial specs are combined in the Op with the outputs from
  +// the host initialization Ops to construct a mapping from full TPU device
  // specs to global TPU ids. Has a single Tensor output which is a
  // matrix of int32 indicating, for each host (outer dimension) and for
  // each TPU on the host (inner dimension) what that TPU's global id
  @@ -108,7 +106,6 @@ in a host.
  REGISTER_OP("_WaitForDistributedTPU")
  .Input("inputs: N * int32")
  .Output("topology: string")
  - .Attr("host_specs: list(string)")
  .Attr("startup_timeout_sec: int = 20")
  .Attr("N: int")
  .SetIsStateful()
  