title: How freeze training of particular layer after particular epoches
body: <p>I want to freeze training of first two layers of following code after 3rd epoch. Total epoch is set to 10.</p>
  
  <pre><code>model = Sequential()
  model.add(Conv2D(32, kernel_size=(3, 3),
                   activation='relu',
                   input_shape=input_shape))
  model.add(Conv2D(64, (3, 3), activation='relu'))
  model.add(MaxPooling2D(pool_size=(2, 2)))
  model.add(Dropout(0.25))
  model.add(Flatten())
  model.add(Dense(128, activation='relu'))
  model.add(Dropout(0.5))
  model.add(Dense(num_classes, activation='softmax'))
  </code></pre>