'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):

    self.minibatches = callback_data['config'].attrs['total_minibatches']
    hist_grp = callback_data.create_group('hist')
    hist_grp.attrs['bins'] = self.be.hist_bins
    hist_grp.attrs['offset'] = self.be.hist_offset
    hist_grp.attrs['time_markers'] = ('minibatch' if self.plot_per_mini else 'epoch')
    hist_grp.attrs['time_steps'] = (self.minibatches if self.plot_per_mini else epochs)
