message: Assert the output of RNNCell.__call__ is consistent with RNNCell.output_size
  This is a potentially breaking change for subclasses of RNNCell with inconsistent
  __call__/output_size. However, the fix is always trivial.
diff: @@ -256,6 +256,7 @@ def _rnn_step(
  """Run RNN step. Pass through either no or some past state."""
  new_output, new_state = call_cell()
  + nest.assert_same_structure(zero_output, new_output)
  nest.assert_same_structure(state, new_state)
  flat_new_state = nest.flatten(new_state)
  @@ -274,6 +275,7 @@ def _rnn_step(
  # steps. This is faster when max_seq_len is equal to the number of unrolls
  # (which is typical for dynamic_rnn).
  new_output, new_state = call_cell()
  + nest.assert_same_structure(zero_output, new_output)
  nest.assert_same_structure(state, new_state)
  new_state = nest.flatten(new_state)
  new_output = nest.flatten(new_output)
  