message: Flatten the call graph after recomputation
  Summary:
  When we recompute, some ops might call other computations (example
  serialized graphs)
  Ref T18690
  Test Plan: All tests pass
  Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, davidn
  Maniphest Tasks: T18690
diff: @@ -842,6 +842,9 @@ StatusOr<std::unique_ptr<Executable>> PoplarCompiler::RunBackend(
  pipeline.AddPass<PipelineRecomputation>(
  poplar_executor->RecomputationEnabled(),
  poplar_executor->StatefulRecomputationEnabled());
  + if (poplar_executor->RecomputationEnabled()) {
  + pipeline.AddPass<FlattenCallGraph>();
  + }
  pipeline.AddPass<HloDCE>();
  // Beyond this point non of the passes in the pipeline are allowed to modify
  // the instructions in the HloModule.
  