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

def _create_placeholders(self, n_features, n_classes):

    self.input_data = tf.placeholder(tf.float32, [None, n_features], name='x-input')
    self.input_labels = tf.placeholder(tf.float32, [None, n_classes], name='y-input')
    self.keep_prob = tf.placeholder(tf.float32, name='keep-probs')
