'Called when training is about to begin
Arguments:
callback_data (HDF5 dataset): shared data between callbacks
model (Model): model object
epochs (int): Total epochs'

def on_train_begin(self, callback_data, model, epochs):

    points = callback_data['config'].attrs['total_minibatches']
    callback_data.create_dataset('gan/gen_iter', (points,))
    callback_data.create_dataset('gan/cost_dis', (points,))
    callback_data['gan/gen_iter'].attrs['time_markers'] = 'minibatch'
    callback_data['gan/cost_dis'].attrs['time_markers'] = 'minibatch'
