class_docstr: Callback function used to calculate the NDCG@k metric of validation set at the end of each epoch.
  Weights of the model with the highest NDCG@k value is saved.
docstr: Returns a list of the NDCG@k of the validation set metrics calculated
  at the end of each epoch.
code: def get_data(self):
      """Returns a list of the NDCG@k of the validation set metrics calculated
      at the end of each epoch."""
      return self._data
  
imports: import numpy
  import matplotlib.pyplot
  import seaborn
  from recommenders.evaluation.python_evaluation import ndcg_at_k
  import tensorflow
  from tensorflow.keras.layers import *
  from tensorflow.keras.models import Model
  from tensorflow.keras import backend
  from tensorflow.keras.callbacks import ReduceLROnPlateau, Callback
label_desc: Use image node_id 5 for calling the Metrics obj's underlying member method code with example usage: obj.get_data() and returns: self