(int or tuple): Padding that was added to the input

    Inputs:
        - `input`: the input Tensor to invert
        - `indices`: the indices given out by :class:`~torch.nn.MaxPool3d`
        - `output_size` (optional): the targeted output size

    Shape:
        - Input: :math:`(N, C, D_{in}, H_{in}, W_{in})` or :math:`(C, D_{in}, H_{in}, W_{in})`.
        - Output: :math:`(N, C, D_{out}, H_{out}, W_{out})` or :math:`(C, D_{out}, H_{out}, W_{out})`, where

          .. math::
              D_{out} = (D_{in} - 1) \times \text{stride[0]} - 2 \times \text{padding[0]} + \text{kernel\_size[0]}

          .. math::
              H_{out} = (H_{in} - 1) \times \text{stride[1]} - 2 \times \text{padding[1]} + \text{kernel\_size[1]}

          .. math::
              W_{out} = (W_{in} - 1) \times \text{stride[2]} - 2 \times \text{padding[2]} + \text{kernel\_size[2]}

          or as given by :attr:`output_size` in the call operator

    Example::

        >>> # pool of square window of size=3, stride=2
        >>> pool = nn.MaxPool3d(3, stride=2, return_indices=True)
        >>> unpool = nn.MaxUnpool3d(3, stride=2)
        >>> output, indices = pool(torch.randn(20, 16, 51, 33, 15))
        >>> unpooled_output = unpool(output, indices)
        >>> unpooled_output.size()
        torch.Size([20, 16, 51, 33, 15])
    r)