title: TensorBoard Callback in Keras does not respect initial_epoch of fit?
body: <p>
  I'm trying to train multiple models in parallel on a single graphics card. To achieve that I need to resume training of models from saved weights which is not a problem. The <code>model.fit()</code> method has even a parameter initial_epoch that lets me tell the model which epoch the loaded model is on. However when i pass a TensorBoard callback to the <code>fit()</code> method in order to monitor the training of the models, on Tensorboard all data is shown on x=0.</p>
  
  <p>Is there a ways to overcome this and adjust the epoch on tensorboard?</p>
  
  <p>By the way: Im running Keras 2.0.6 and Tensorflow 1.3.0.
  </p>
  
  <pre class="lang-python prettyprint-override"><code>self.callbacks = [TensorBoardCallback(log_dir='./../logs/'+self.model_name, histogram_freq=0, write_graph=True, write_images=False, start_epoch=self.step_num)]
  self.model.fit(x=self.data['X_train'], y=self.data['y_train'], batch_size=self.input_params[-1]['batch_size'], epochs=1, validation_data=(self.data['X_test'], self.data['y_test']), verbose=verbose, callbacks=self.callbacks, shuffle=self.hyperparameters['shuffle_data'], initial_epoch=self.step_num)
  self.model.save_weights('./weights/%s.hdf5'%(self.model_name))
  self.model.load_weights('./weights/%s.hdf5'%(self.model_name))
  self.model.fit(x=self.data['X_train'], y=self.data['y_train'], batch_size=self.input_params[-1]['batch_size'], epochs=1, validation_data=(self.data['X_test'], self.data['y_test']), verbose=verbose, callbacks=self.callbacks, shuffle=self.hyperparameters['shuffle_data'], initial_epoch=self.step_num)
  self.model.save_weights('./weights/%s.hdf5'%(self.model_name))
  </code></pre>
  
  <p>
  The resulting graph on Tensorboard looks like this which is not what i was hoping for:
  <a href="https://i.stack.imgur.com/iWRh5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/iWRh5.png" alt="enter image description here"></a>
  </p>
  
  <p><strong>Update:</strong></p>
  
  <p>When passing <code>epochs=10</code> to the first <code>model.fit()</code> the 10 epoch results are displayed in TensorBoard (see picture).</p>
  
  <p>However when reloading the model and running it (with the same callback attached) the <code>on_epoch_end</code> method of the callback gets never called.</p>
  
  <p><a href="https://i.stack.imgur.com/xaCVy.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/xaCVy.png" alt="enter image description here"></a></p>