
outer_jacobian_strategy (str, optional) -- The
        Hessian is computed by computing the Jacobian of a Jacobian.
        The inner Jacobian is always computed in reverse-mode AD.
        Setting strategy to ""forward-mode"" or ""reverse-mode""
        determines whether the outer Jacobian will be computed with
        forward or reverse mode AD. Currently, computing the outer
        Jacobian in ""forward-mode"" requires "vectorized=True".
        Defaults to ""reverse-mode"".

Returns:
      if there is a single input, this will be a single Tensor
      containing the Hessian for the input. If it is a tuple, then the
      Hessian will be a tuple of tuples where "Hessian[i][j]" will
      contain the Hessian of the "i"th input and "j"th input with size
      the sum of the size of the "i"th input plus the size of the
      "j"th input. "Hessian[i][j]" will have the same dtype and device
      as the corresponding "i"th input.
Return type: