title: how to use tensorlayer about using tensorboard?
body: <p>I find that tensorlayer's API fit has an option of tensorboard,while i set it true. i also tried to  tf.summry my loss ,why there is nothing when i open tensorboard?
  there are some of my codes:</p>
  
  <pre><code>with tf.name_scope('loss'):
      cost = tl.cost.cross_entropy(y,y_,name='softmaxloss')
      tf.summary.scalar("loss", cost)
  
  with tf.name_scope('accuracy'):
      correct_pred = tf.equal(tf.argmax(y,1),y_)
      acc = tf.reduce_mean(tf.cast(correct_pred,tf.float32))
      tf.summary.scalar("accuracy", acc)
  
  tl.utils.fit(sess,network,train_op,cost,X_train,y_train,x,y_,acc=acc,batch_size=256,
               n_epoch=200,print_freq=1,X_val=X_val, y_val=y_val,eval_train=False,tensorboard=True)
  </code></pre>
  
  <p>and i really dont know how to viluializes it</p>