(hidden_size)
Note:
 All the weights and biases are initialized from
 \mathcal{U}(-\sqrt{k}, \sqrt{k}) where k =
 \frac{1}{\text{hidden\_size}}

Examples:
  >>> rnn = nn.RNNCell(10, 20)
  >>> input = torch.randn(6, 3, 10)
  >>> hx = torch.randn(3, 20)
  >>> output = []
  >>> for i in range(6):
  ...     hx = rnn(input[i], hx)
  ...     output.append(hx)
