message: add json serialization for dimension value.
diff: @@ -110,6 +110,9 @@ def get_json_type(obj):
  if type(obj).__name__ == type.__name__:
  return obj.__name__
  + if isinstance(obj, tensor_shape.Dimension):
  + return obj.value
  +
  if isinstance(obj, tensor_shape.TensorShape):
  return obj.as_list()
  