Parameters:
    func (function): a Python function that takes a Tensor or a Tensor
        list/tuple as inputs and returns a Tensor with a single element.
    inputs (Tensor|list(Tensor)|tuple(Tensor)): the input Tensor or 
        Tensor list/tuple of the function ``func``.
    v (Tensor|list(Tensor)|tuple(Tensor)|None, optional): the vector used
        to compute vector hessian product. ``v`` should have same shape
        and dtype with ``inputs``. If ``v`` is None, it will be set as
        Tensor|list(Tensor) with all elements 1. Defaults to "None".
    create_graph (bool, optional): whether to create the gradient graphs
        of the computing process. When it is True, higher order derivatives
        are supported to compute; when it is False, the gradient graphs of
        the computing process would be discarded. Defaults to ``False``.
    allow_unused (bool, optional): whether to raise error or return None if
        some Tensors of `inputs` are unreachable in the graph. Error would
        be raised if allow_unused=False, and None would be returned as
        their gradients if allow_unused=True. Default False.
Returns:
    output (tuple): tuple with:
        func_output (Tensor): output of ``func(inputs)``
        vhp (list(Tensor)): result of the vector hessian product
        with the same shape and dtype as the inputs.