message: Add VLOG debug statement for Save and Restore in DatasetBaseIterator
diff: @@ -680,7 +680,7 @@ class IteratorBase {
  IteratorContext* ctx, model::Node::Args args) const = 0;
  // Restores the state of this iterator.
  - Status Restore(IteratorContext* ctx, IteratorStateReader* reader) {
  + virtual Status Restore(IteratorContext* ctx, IteratorStateReader* reader) {
  int64 start_us = EnvTime::NowMicros();
  TF_RETURN_IF_ERROR(RestoreInternal(ctx, reader));
  VLOG(1) << "Restored " << prefix() << " in "
  @@ -693,7 +693,7 @@ class IteratorBase {
  Status SaveInput(SerializationContext* ctx, IteratorStateWriter* writer,
  const std::unique_ptr<IteratorBase>& input) {
  int64 start_us = EnvTime::NowMicros();
  - TF_RETURN_IF_ERROR(input->SaveInternal(ctx, writer));
  + TF_RETURN_IF_ERROR(input->Save(ctx, writer));
  VLOG(2) << "Saved " << input->prefix() << " in "
  << (EnvTime::NowMicros() - start_us) << "us";
  return Status::OK();
  @@ -704,7 +704,7 @@ class IteratorBase {
  Status RestoreInput(IteratorContext* ctx, IteratorStateReader* reader,
  const std::unique_ptr<IteratorBase>& input) {
  int64 start_us = EnvTime::NowMicros();
  - TF_RETURN_IF_ERROR(input->RestoreInternal(ctx, reader));
  + TF_RETURN_IF_ERROR(input->Restore(ctx, reader));
  VLOG(2) << "Restored " << input->prefix() << " in "
  << (EnvTime::NowMicros() - start_us) << "us";
  return Status::OK();
  @@ -1003,10 +1003,18 @@ class DatasetBaseIterator : public IteratorBase {
  int* num_skipped) final;
  Status Save(SerializationContext* ctx, IteratorStateWriter* writer) final {
  + VLOG(2) << "Attempting to save checkpoints on iterator (prefix: "
  + << prefix() << ") from " << dataset()->DebugString();
  return IteratorBase::Save(ctx, writer);
  }
  protected:
  + Status Restore(IteratorContext* ctx, IteratorStateReader* reader) final {
  + VLOG(2) << "Attempting to restore checkpoints on iterator (prefix: "
  + << prefix() << ") from " << dataset()->DebugString();
  + return IteratorBase::Restore(ctx, reader);
  + }
  +
  // Internal implementation of GetNext that is wrapped in tracing logic.
  virtual Status GetNextInternal(IteratorContext* ctx,
  std::vector<Tensor>* out_tensors,
  