message: Register compilation kernels before validating graph
  Normally MarkForCompilationPass calls XlaOpRegistry::RegisterCompilationKernels
  and ValidateGraph is runs after that so calling it again is unnecessary.
  However in some cases ValidateGraph can be called without a prior call to
  MarkForCompilationPass and so we call RegisterCompilationKernels in
  ValidateGraph to be sure.
diff: @@ -1046,6 +1046,10 @@ Status GetPotentialFunctionName(const Node& node, const string** name) {
  Status ValidateGraph(const Graph* graph,
  const FunctionLibraryDefinition& flib_def,
  const DeviceType& device_type, const string& name) {
  + // Make sure the XLA compilation kernels are registered. This operation is
  + // idempotent so it is fine if someone called it already.
  + XlaOpRegistry::RegisterCompilationKernels();
  +
  auto maybe_error = [&](const Node* node, const Status& s) -> Status {
  if (!s.ok()) {
  return errors::InvalidArgument(absl::StrCat(
  