feat: allow nested list/tuple input/output and None type

[cijinsama]

Main change features:

• Allow nested list/tuple input/output like

    def forward(self, x: torch.Tensor, y: List[Tuple[torch.Tensor, torch.Tensor, ...]]):
        ...
        return c, [(mean_1, var_1, weight_1), (mean_2, var_2, weight_2), ...]

• Allow input/output with None in list/tuple

[willyfh]

I left few comments in the file changes. In addition, is it appropriate to use a public function instead of the private one? (Using _ in front of the function name)