I saw that in the code, hidden_state is not implemented: ``` def forward(self, input_tensor, hidden_state=None): """ Parameters ---------- input_tensor: todo 5-D Tensor either of shape (t, b, c, h, w) or (b, t, c, h, w) hidden_state: todo None. todo implement stateful ``` meanwhile, hidden_dim is given. What is the difference between those two variables?