title: What does "ValueError: When feeding symbolic tensors to a model, we expect the tensors to have a static batch size" mean?
body: <p>I am using keras to build a neural network for predicting diabetes. However I encountered a ValueError: When feeding symbolic tensors to a model, we expect the tensors to have a static batch size.</p>
  
  <p>I tried changing the input shapes but I am still stuck.</p>
  
  <pre><code>num_classes = 2
  from keras.layers import Input, Dense
  from keras.models import Model
  
  # This returns a tensor
  inputs = Input(shape=(784,))
  
  # a layer instance is callable on a tensor, and returns a tensor
  x = Dense(64, activation='relu')(inputs)
  x = Dense(64, activation='relu')(x)
  predictions = Dense(10, activation='sigmoid')(x)
  
  # This creates a model that includes
  # the Input layer and three Dense layers
  model = Model(inputs=inputs, outputs=predictions)
  model.compile(optimizer='rmsprop',
            loss='binary_crossentropy',
            metrics=['accuracy'])
  model.fit(x,y)  # starts training
  </code></pre>
  
  <p>After running
  ValueError: When feeding symbolic tensors to a model, we expect the tensors to have a static batch size.</p>