I think the fundamental reason is that people tend to use the functional interface for that… But you could easily do this yourself as
class ReflectionPad3d(torch.nn.modules.padding._ReflectionPadNd):
    def __init__(self, padding):
        super(ReflectionPad3d, self).__init__()
        self.padding = _ntuple(6)(padding)

or so.
Best regards
Thomas