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."""
    core_initial_state = self._core.initial_state(
        batch_size, dtype=dtype, trainable=trainable,
        trainable_initializers=trainable_initializers,
        trainable_regularizers=trainable_regularizers, name=name)

    dropout_masks = [None] * len(self._dropout_state_size)
    def set_dropout_mask(index, state, keep_prob):
      if index is not None:
        ones = tf.ones_like(state, dtype=dtype)
        dropout_masks[index] = tf.nn.dropout(ones, keep_prob=keep_prob)
    tf.contrib.framework.nest.map_structure(
        set_dropout_mask,
        self._dropout_indexes, core_initial_state, self._keep_probs)

    return core_initial_state, dropout_masks