message: Allow unit-length input for Concatenate layer
diff: @@ -489,9 +489,9 @@ class Concatenate(_Merge):
  @tf_utils.shape_type_conversion
  def build(self, input_shape):
  # Used purely for shape validation.
  - if not isinstance(input_shape[0], tuple) or len(input_shape) < 2:
  + if not isinstance(input_shape[0], tuple) or len(input_shape) < 1:
  raise ValueError('A `Concatenate` layer should be called '
  - 'on a list of at least 2 inputs')
  + 'on a list of at least 1 input.')
  if all(shape is None for shape in input_shape):
  return
  reduced_inputs_shapes = [list(shape) for shape in input_shape]
  