Provide a warning that get_variables on Sequential always returns ().

def get_variables(self, *args, **kwargs):
    """Provide a warning that get_variables on Sequential always returns ()."""
    tf.logging.warning(
        "Calling Sequential.get_variables, which will always return an empty "
        "tuple. get_variables() can only return variables created directly by "
        "a Module, or created by submodules directly created inside the "
        "Module. Sequential is constructed from already constructed submodules "
        "and so this will always be empty. See the documentation for more "
        "details, but tl;dr if you need to connect some modules sequentially "
        "and call get_variables on the result, writing a simple custom module "
        "is the simplest way. Another option is to call get_all_variables().")
    return super(Sequential, self).get_variables(*args, **kwargs)