message: Fix verifier for gradient accumulation
  Summary:
  This resolves problems on the TF2.4 branch.
  Ref T31435
  Test Plan: CI
  Reviewers: jakeh, #tensorflow, simonl, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved
  Maniphest Tasks: T31435
diff: @@ -243,7 +243,7 @@ Status GradientAccumulationVerifier::VerifyGenericGradientAccumulation(
  inst->user_count(), " users.");
  }
  HloInstruction* user = inst->users()[0];
  - do {
  + while (!IsPoplarInstruction(PoplarOp::GradientAccumulatorSink)(user)) {
  if (user->user_count() != 1) {
  return InternalErrorStrCat(
  "Expected the gradient accumulation buffer to be used "
  @@ -272,7 +272,7 @@ Status GradientAccumulationVerifier::VerifyGenericGradientAccumulation(
  (*inplace_modifier)->ToString(), ".");
  }
  user = next_user;
  - } while (!IsPoplarInstruction(PoplarOp::GradientAccumulatorSink)(user));
  + }
  // Make sure the sink is only used by a resource update.
  if (user->user_count() != 1) {
  @@ -398,7 +398,6 @@ Status GradientAccumulationVerifier::VerifyGenericGradientAccumulation(
  StatusOr<bool> GradientAccumulationVerifier::Run(HloModule* module) {
  std::unique_ptr<CallGraph> call_graph = CallGraph::Build(module);
  -
  for (HloComputation* comp : module->MakeComputationPostOrder()) {
  if (IsPopOpsFusion(comp)) {
  continue;
  