message: Add a target for creating an IPU XLA service executable
  Summary:
  This might be the future of large rackscale systems.  Useful even
  if not, because it forms the basis for part of the MLIR migration
  path.
  Test Plan: All tests ok.
  Reviewers: #tensorflow, georgep
diff: @@ -6,6 +6,7 @@ load("//tensorflow/compiler/xla/tests:build_defs.bzl", "xla_test")
  load("@local_config_poplar//poplar:build_defs.bzl", "poplar_lib_directory")
  load("@local_config_poplar//poplar:build_defs.bzl", "poplibs_lib_directory")
  load("@local_config_poplar//poplar:build_defs.bzl", "tf_poplar_build_tag")
  +load("//tensorflow:tensorflow.bzl", "tf_cc_binary")
  load("//tensorflow:tensorflow.bzl", "tf_cc_test")
  load("//tensorflow:tensorflow.bzl", "tf_py_test")
  load("//tensorflow:tensorflow.bzl", "tf_gen_op_wrapper_py")
  @@ -629,6 +630,14 @@ cc_library(
  alwayslink = True,
  )
  +tf_cc_binary(
  + name = "grpc_service_main_ipu",
  + deps = [
  + ":poplar_lib",
  + "//tensorflow/compiler/xla/rpc:grpc_service_main_library",
  + ],
  +)
  +
  cc_library(
  name = "test_utils",
  srcs = [],
  