message: Only build RNN cells if they haven't already been built.
diff: @@ -144,6 +144,7 @@ class StackedRNNCells(Layer):
  input_shape = input_shape[0]
  for cell in self.cells:
  if isinstance(cell, Layer):
  + if not cell.built:
  cell.build(input_shape)
  if getattr(cell, 'output_size', None) is not None:
  output_dim = cell.output_size
  @@ -537,6 +538,7 @@ class RNN(Layer):
  # allow cell (if layer) to build before we set or validate state_spec
  if isinstance(self.cell, Layer):
  + if not self.cell.built:
  self.cell.build(step_input_shape)
  # set or validate state_spec
  