message: Don't generate the mapping Json unless requested
  Summary:
  Small improvment for T27045
  Ref T27045
  Test Plan: CI
  Reviewers: jackh, alfiee, markf, #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved
  Maniphest Tasks: T27045
diff: @@ -1415,9 +1415,6 @@ StatusOr<std::unique_ptr<Executable>> PoplarCompiler::RunBackend(
  try {
  VLOG(1) << "Begin compiling Poplar engine " << module->name();
  - map_json = GetTensorMappingJson(module->name(), main_graph,
  - resources.tensor_maps);
  -
  auto progress_logging = [](int progress, int total) {
  float progress_percent = std::floor(
  100.0f * static_cast<float>(progress) / static_cast<float>(total));
  @@ -1508,6 +1505,9 @@ StatusOr<std::unique_ptr<Executable>> PoplarCompiler::RunBackend(
  TF_ASSIGN_OR_RETURN(auto inst_info,
  GetInstructionCompilationInfo(module, resources));
  + map_json = GetTensorMappingJson(module->name(), main_graph,
  + resources.tensor_maps);
  +
  poplar_executor->AddCompileEndEventRecord(
  module->name(), report_stream.str(), graph_stream.str(), map_json,
  inst_info, tensorflow_info, duration);
  