message: Add an environment variable to dump the vertex graph
  Summary: Add an environment variable for getting at the vertex graph
  Test Plan: None
  Reviewers: georgep
diff: @@ -485,6 +485,12 @@ StatusOr<std::unique_ptr<Executable>> PoplarCompiler::RunBackend(
  progs.push_back(visitor.host_to_device);
  progs.push_back(visitor.device_to_host);
  + char* vertex_filename = getenv("TF_DUMP_VERTEX_GRAPH");
  + if (vertex_filename) {
  + std::ofstream stream(vertex_filename);
  + graph.outputVertexGraph(stream, progs);
  + }
  +
  if (visitor.all_outputs_are_parameters) {
  VLOG(1) << "Skip engine compilation - all outputs are inputs";
  } else {
  