class_docstr: Multinomial Variational Autoencoders (Multi-VAE) for Collaborative Filtering implementation
  
  :Citation:
  
      Liang, Dawen, et al. "Variational autoencoders for collaborative filtering."
      Proceedings of the 2018 World Wide Web Conference. 2018.
      https://arxiv.org/pdf/1802.05814.pdf
docstr: Returns the list of NDCG@k at each epoch.
code: def ndcg_per_epoch(self):
      """Returns the list of NDCG@k at each epoch."""
      return self.val_ndcg
  
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 10 for calling the Mult_VAE obj's underlying member method code with example usage: obj.ndcg_per_epoch() and returns: self