Functional forms of splitters

[SteshinSS]

Just a half-baked thought from the #9 discussion: It might be useful to have splitters in functional forms similar to torch.nn.functional.

Most of the splitters can be stateless, so we could create functions that create a splitter object, call .split(), and then return the results. This could simplify usage, but it would create an additional interface, which is not in accordance with PEP 20.