'Create the TensorFlow variables for the model.
:return: self'

def _create_variables(self, n_features, W_=None, bh_=None, bv_=None):

    if W_:
        self.W_ = tf.Variable(W_, name='enc-w')
    else:
        self.W_ = tf.Variable(tf.truncated_normal(shape=[n_features, self.n_components], stddev=0.1), name='enc-w')
    if bh_:
        self.bh_ = tf.Variable(bh_, name='hidden-bias')
    else:
        self.bh_ = tf.Variable(tf.constant(0.1, shape=[self.n_components]), name='hidden-bias')
    if bv_:
        self.bv_ = tf.Variable(bv_, name='visible-bias')
    else:
        self.bv_ = tf.Variable(tf.constant(0.1, shape=[n_features]), name='visible-bias')
