human
Complete the below
def dump_graph(root_name, out_name='cg.dot', variable_style=None, function_style=None):

    ''''Returns a trainer extension to dump a computational graph.
    This extension dumps a computational graph. The graph is output in DOT
    language.
    It only dumps a graph at the first invocation.
    .. note::
    As of v2.0.0, the computational graph is not kept by default. This
    extension changes this behavior until the first invocation. **It is
    strongly recommended to use it with the default trigger setting.**
    The detailed behavior of this extension since v2.0.0 is as follows.
    1. In its initializer, it turns on the
    ``chainer.config.keep_graph_on_report`` flag.
    2. At the first iteration, it dumps the graph using the graph held by
    the reported variable.
    3. After dumping the graph, it turns off the flag (if it was originally
    turned off) so that any variable reported afterward does not hold
    a computational graph.
    When the ``keep_graph_on_report`` flag is turned on, the computational
    graph created by the updater is kept during the invocation of
    extensions. It will cause an unnecessarily large memory consumption
    when an extension also uses a large amount of memory, e.g.
    :class:`~chainer.training.extensions.Evaluator`.
    With the default setting, the ``dump_graph`` extension is called at the
    first iteration. Since :class:`~chainer.training.extensions.Evaluator`
    is not called at the first iteration in most cases, it does not cause
    any memory problem.
    Args:
    root_name (str): Name of the root of the computational graph. The
    root variable is retrieved by this name from the observation
    dictionary of the trainer.
    out_name (str): Output file name.
    variable_style (dict): Dot node style for variables. Each variable is
    rendered by an octagon by default.
    function_style (dict): Dot node style for functions. Each function is
    rendered by a rectangular by default.
    .. seealso::
    See :func:`~chainer.computational_graph.build_computational_graph`
    for the ``variable_style`` and ``function_style`` arguments.'
    '''
 What is a the Python program you requested?
assistant
#Here is the Python program you requested.
    def trigger(trainer):
        return (trainer.updater.iteration == 1)
    if (variable_style is None):
        variable_style = _var_style
    if (function_style is None):
        function_style = _func_style
    original_flag = [None]
    def initializer(_):
        original_flag[0] = configuration.config.keep_graph_on_report
        configuration.config.keep_graph_on_report = True
    @extension.make_extension(trigger=trigger, initializer=initializer)
    def dump_graph(trainer):
        try:
            var = trainer.observation[root_name]
            if (not isinstance(var, variable.Variable)):
                raise TypeError('root    value    is    not    a    Variable')
            cg = computational_graph.build_computational_graph([var], variable_style=variable_style, function_style=function_style).dump()
            out_path = os.path.join(trainer.out, out_name)
            with open(out_path, 'w') as f:
                f.write(cg)
        finally:
            configuration.config.keep_graph_on_report = original_flag[0]
    return dump_graph