Add missing `override' (#17118)

This fixes a warning produced by clang:
./tensorflow/contrib/tensor_forest/kernels/v4/grow_stats.h:470:8: warning: 'InitLeafClassStats' overrides a member function but is not marked 'override' [-Winconsistent-missing-override]
  void InitLeafClassStats(int best_split_index, LeafStat* left_stats,
       ^
./tensorflow/contrib/tensor_forest/kernels/v4/grow_stats.h:190:16: note: overridden virtual function is here
  virtual void InitLeafClassStats(int best_split_index, LeafStat* left_stats,
               ^