class_docstr: Generalized Matrix Factorization.
  
  Parameters
  ----------
  num_factors: int, optional, default: 8
      Embedding size of MF model.
      
  reg: float, optional, default: 0.
      Regularization (weight_decay).
  
  num_epochs: int, optional, default: 20
      Number of epochs.
  
  batch_size: int, optional, default: 256
      Batch size.
  
  num_neg: int, optional, default: 4
      Number of negative instances to pair with a positive instance.
  
  lr: float, optional, default: 0.001
      Learning rate.
  
  learner: str, optional, default: 'adam'
      Specify an optimizer: adagrad, adam, rmsprop, sgd
  
  backend: str, optional, default: 'tensorflow'
      Backend used for model training: tensorflow, pytorch
      
  early_stopping: {min_delta: float, patience: int}, optional, default: None
      If `None`, no early stopping. Meaning of the arguments: 
      
      - `min_delta`: the minimum increase in monitored value on validation set to be considered as improvement,            i.e. an increment of less than min_delta will count as no improvement.
      
      - `patience`: number of epochs with no improvement after which training should be stopped.
  
  name: string, optional, default: 'GMF'
      Name of the recommender model.
  
  trainable: boolean, optional, default: True
      When False, the model is not trained and Cornac assumes that the model is already         pre-trained.
  
  verbose: boolean, optional, default: False
      When True, some running logs are displayed.
  
  seed: int, optional, default: None
      Random seed for parameters initialization.
  
  References
  ----------
  * He, X., Liao, L., Zhang, H., Nie, L., Hu, X., & Chua, T. S. (2017, April). Neural collaborative filtering.     In Proceedings of the 26th international conference on world wide web (pp. 173-182).
code: def _build_model_pt(self):
      from .backend_pt import GMF
  
      return GMF(self.num_users, self.num_items, self.num_factors)
  
file_path: cornac/cornac/models/ncf/recom_gmf.py
imports: import numpy
  from .recom_ncf_base import NCFBase
  from ...exception import ScoreException
label_desc: Use image node_id 4 for calling the GMF obj's underlying member method code with example usage: obj._build_model_pt() and returns: GMF