title: Keras (tensorflow backend) getting "TypeError: unhashable type: 'Dimension'"
body: <p>Hi I get a dimension error when fitting this model, would anyone know why?    </p>
  
  <pre><code>num_classes = 11
  input_shape = (64,64,1)
  batch_size = 128
  epochs = 12
  X_train = tf.reshape(X_train, [-1, 64, 64, 1])
  X_test = tf.reshape(X_test, [-1, 64, 64, 1])
  
  model = Sequential()
  model.add(Conv2D(32, kernel_size=(3,3), strides=1, activation='relu', input_shape=input_shape)) 
  model.add(Flatten())
  model.add(Dense(128, activation='relu'))
  model.add(Dense(num_classes, activation='softmax'))
  
  model.compile(loss=keras.losses.categorical_crossentropy,
            optimizer=keras.optimizers.Adadelta(),
            metrics=['accuracy'])
  
  model.fit(X_train, y_train,
        batch_size=batch_size,
        epochs=epochs,
        verbose=1,
        validation_data=(X_test, y_test))
  </code></pre>
  
  <p>The dimension of each variable is</p>
  
  <pre><code>X_train = (27367, 64, 64, 1)
  X_test = (4553, 64, 64, 1)
  y_train = (164202, 11)
  y_test = (27318, 11)
  </code></pre>