message: [TF:XLA] Improve readability of HLO graphs when rendered via Tensorboard.
  Add operator metadata around the computation arguments and retvals, so they are grouped together.
  Teach the batchnorm expander pass to propagate the operator metadata from the original batch norm operators.
diff: @@ -153,6 +153,7 @@ Status BatchNormExpanderVisitor::HandleBatchNormTraining(
  std::vector<HloInstruction*> added_instructions;
  auto add = [&](std::unique_ptr<HloInstruction> inst) {
  HloInstruction* added_inst = computation_->AddInstruction(std::move(inst));
  + added_inst->set_metadata(batch_norm->metadata());
  added_instructions.push_back(added_inst);
  return added_inst;
  };
  @@ -334,6 +335,7 @@ Status BatchNormExpanderVisitor::HandleBatchNormInference(
  std::vector<HloInstruction*> added_instructions;
  auto add = [&](std::unique_ptr<HloInstruction> inst) {
  HloInstruction* added_inst = computation_->AddInstruction(std::move(inst));
  + added_inst->set_metadata(batch_norm->metadata());
  added_instructions.push_back(added_inst);
  return added_inst;
  };
  @@ -419,6 +421,7 @@ Status BatchNormExpanderVisitor::HandleBatchNormGrad(
  std::vector<HloInstruction*> added_instructions;
  auto add = [&](std::unique_ptr<HloInstruction> inst) {
  HloInstruction* added_inst = computation_->AddInstruction(std::move(inst));
  + added_inst->set_metadata(batch_norm->metadata());
  added_instructions.push_back(added_inst);
  return added_inst;
  };
  