// First, walk the Ops dependencies.
  GatherOpsForExtraction(&backward_pass_ops, merged_set, /*predecessors=*/false,
                         /*successors=*/true);

  VLOG(3) << "found " << backward_pass_ops.size() << " backwards pass ops";

  // If any inputs are to the backward_pass_ops region are direct
  // TPUReplicatedInput ops, then include (if this is the only use) or

  // First, walk the Ops dependencies.
  GatherOpsForExtraction(&backward_pass_ops, merged_set, /*predecessors=*/false,
                         /*successors=*/true);

  VLOG(3) << "Found " << backward_pass_ops.size() << " backwards pass ops.";

  // If any inputs are to the backward_pass_ops region are direct
  // TPUReplicatedInput ops, then include (if this is the only use) or