Builds the default start state tensor of zeros.

def initial_state(self, batch_size, dtype=tf.float32, trainable=False,
                    trainable_initializers=None, trainable_regularizers=None,
                    name=None):
    """Builds the default start state tensor of zeros."""
    return self._core.initial_state(
        batch_size, dtype=dtype, trainable=trainable,
        trainable_initializers=trainable_initializers,
        trainable_regularizers=trainable_regularizers, name=name)