message: Return the empty sequence for an outfeed with synthetic data
  Summary:
  Creation function requires a sequence for any successful operation,
  so return an empty one in the case of synthetic data.
  Test Plan: All tests ok.
  Reviewers: #tensorflow, georgep
diff: @@ -183,7 +183,7 @@ StatusOr<poplar::program::Program> CreateOutfeed(CompilerResources& res,
  }
  if (UseSyntheticData()) {
  - return Status::OK();
  + return seq;
  }
  HloInstruction* operand = outfeed->operands()[0];
  