'Create the TensorFlow variables for the model.
:param n_features: number of features
:param n_classes: number of classes
:return: self'

def _create_variables(self, n_features, n_classes):

    self.W_ = tf.Variable(tf.zeros([n_features, n_classes]), name='weights')
    self.b_ = tf.Variable(tf.zeros([n_classes]), name='biases')
