'Build a tf.placeholder from the metadata, with an optional prefix.
Args:
prefix(str): the name of the placeholder will be ``prefix + self.name``
Returns:
tf.Tensor:'

def build_placeholder(self, prefix=''):

    with tf.name_scope(None):
        ret = tf.placeholder(self.type, shape=self.shape, name=(prefix + self.name))
    if ((prefix == '') and (self._cached_placeholder is None)):
        self._cached_placeholder = ret
    return ret
