title: Resource localhost/total/N10tensorflow3VarE does not exist
body: <p>I'm working with Google Colab and trying to train a model using VGG blocks. Like this:</p>
  
  <pre><code>METRICS = [
        keras.metrics.TruePositives(name='tp'),
        keras.metrics.FalsePositives(name='fp'),
        keras.metrics.TrueNegatives(name='tn'),
        keras.metrics.FalseNegatives(name='fn'), 
        keras.metrics.BinaryAccuracy(name='accuracy'),
        keras.metrics.Precision(name='precision'),
        keras.metrics.Recall(name='recall'),
        keras.metrics.AUC(name='auc'),
  ]
  
  # function for creating a vgg block
  def vgg_block(layer_in, n_filters, n_conv):
    # add convolutional layers
    for _ in range(n_conv):
      layer_in = Conv2D(n_filters, (3,3), padding='same', activation='relu')(layer_in)
    # add max pooling layer
    layer_in = MaxPooling2D((2,2), strides=(2,2))(layer_in)
    return layer_in
  
  # define model input
  visible = Input(shape=(256, 256, 3))
  # add vgg module
  layer = vgg_block(visible, 64, 2)
  
  #####################################
  
  
  flat = Flatten()(layer)
  hidden1 = Dense(128, activation='relu')(flat)
  output = Dense(1, activation='sigmoid')(hidden1)
  model = Model(inputs=visible, outputs=output)
  print(model.summary())
  
  # plot model architecture
  plot_model(model, show_shapes=True, to_file='vgg_block.png')
  
  model.compile(loss='binary_crossentropy',
              optimizer='rmsprop',
              metrics=METRICS)
  
  # New lines to obtain the best model in term of validation accuracy
  from keras.callbacks import ModelCheckpoint
  filepath="weights-improvement-{epoch:02d}-{val_accuracy:.2f}.h5"
  checkpoint = ModelCheckpoint(filepath, monitor='val_accuracy', verbose=1, save_best_only=True, mode='max')
  callbacks_list = [checkpoint]
  
  </code></pre>
  
  <p>But, when I try to use model.fit_generator it gives me an error. The code I'm using is:</p>
  
  <pre><code>history = model.fit_generator(
      train_generator,
      steps_per_epoch=2000 // batch_size,
      epochs=20,
      validation_data=validation_generator,
      validation_steps=800 // batch_size,
      callbacks=callbacks_list
  )
  </code></pre>
  
  <p>I have tried everything and I don't know what to do. It gives me the following error:</p>
  
  <pre><code>NotFoundError: 2 root error(s) found.
    (0) Not found: Resource localhost/total/N10tensorflow3VarE does not exist.
       [[{{node metrics/accuracy/AssignAddVariableOp}}]]
       [[metrics/precision/Mean/_87]]
    (1) Not found: Resource localhost/total/N10tensorflow3VarE does not exist.
       [[{{node metrics/accuracy/AssignAddVariableOp}}]]
  0 successful operations.
  0 derived errors ignored.
  </code></pre>
  
  <p>I would appreciate any help. I'm kind of new here. What could I do? Thanks!</p>