Get Keras model input from inside a custom callback

I have a very simple question. I have a Keras model (TF backend) defined for classification. I want to dump the training images fed into my model during training for debugging purposes. I am trying to create a custom callback that writes Tensorboard image summaries for this.

But how can I obtain the real training data inside the callback?

Currently I am trying this:

class TensorboardKeras(Callback):                                                                                                                                                                                                                                     
    def __init__(self, model, log_dir, write_graph=True):                                                                                                                                                                                                             
        self.model = model                                                                                                                                                                                                                                            
        self.log_dir = log_dir                                                                                                                                                                                                                                        
        self.session = K.get_session()                                                                                                                                                                                                                                

        tf.summary.image('input_image', self.model.input)                                                                                                                                                                                                             
        self.merged = tf.summary.merge_all()                                                                                                                                                                                                                          

        if write_graph:                                                                                                                                                                                                                                               
            self.writer = tf.summary.FileWriter(self.log_dir, K.get_session().graph)                                                                                                                                                                                  
        else:                                                                                                                                                                                                                                                         
            self.writer = tf.summary.FileWriter(self.log_dir)

    def on_batch_end(self, batch, logs=None):
        summary = self.session.run(self.merged, feed_dict={})                                                                                                                                                                                                         
        self.writer.add_summary(summary, batch)                                                                                                                                                                                                                       
        self.writer.flush()

But I am getting the error: InvalidArgumentError (see above for traceback): You must feed a value for placeholder tensor 'input_1' with dtype float and shape [?,224,224,3]

There must be a way to see what models, get as an input, right?

Or maybe I should try another way to debug it?

from Get Keras model input from inside a custom callback